“contributed all she had, her whole livelihood.”

The poor widow is a good example of what it means to be a member of the Mystical Body of Christ. As stated in the Link to Liturgy Lesson Christian Charity, “The unity of the Mystical Body produces and stimulates charity among the faithful: ‘From this it follows that if one member suffers anything, all the members suffer with him, and if one member is honored, all the members together rejoice.’”  [1]

What is the Mystical Body of Christ? “Naturally the Church is not His physical body, for that is already in Heaven with the Father.” [2] The Mystical Body of Christ is the Catholic Church which was established by Christ as an extension and continuation of the Incarnation. The Mystical Body of Christ is made up of the members of the faithful who by their baptism are united with Christ. “In the words of Pius XII, ‘If we would define and describe the true Church of Jesus Christ – which is the one, holy, Catholic, apostolic Roman Church – we shall find nothing more noble, more sublime, or more divine than the expression ‘the Mystical Body of Jesus Christ’ – an expression that flows spontaneously from the repeated teaching of the sacred Scriptures and the holy Fathers.’” [3]

The Mystical Body of Christ imitates the Physical Body of Christ, thus living out the life of Christ on earth. The Mystical Body of Christ can produce and stimulate charity because Jesus Christ Himself while on earth produced and stimulated charity. His Physical Body produces and stimulates charity most perfectly on the Cross. “I have told you this so that my joy may be in you and your joy may be complete. This is my commandment; love one another as I love you. No one has greater than this, to lay down one’s life for one’s friends. You are my friends if you do what I command you.” [4]
 
The poor widow “stimulates charity among the faithful” and Jesus uses this woman as an example because “she, from her poverty, has contributed all she had, her whole livelihood”. The woman is a true Christian, through her faith she is living the life of Christ, who from His poverty, contributed all He had, giving His whole livelihood. Jesus Christ, “emptied himself, taking the form of a slave, coming in human likeness, and found human in appearance, he humbled himself, becoming obedient to death, even death on a cross. Because of this, God greatly exalted him and bestowed on him the name that is above every name…” [5]
 
Besides producing and stimulating charity, what other things does the Mystical Body do? The Mystical Body does all that the body of Christ did. The physical body of Christ suffered, died and rose again, the mystery of our faith. The physical Body of Christ no longer suffers, but the Mystical Body of Christ, which is made up of the members of the Church, do suffer now, and by dying in Christ have the hope of also rising in Him.  This is why Saint Paul rejoices in his suffering and in fact, why all Christians may rejoice in suffering. “Now I rejoice in my sufferings for your sake, and in my flesh I am filling up what is lacking in the afflictions of Christ on behalf of his body, which is the church.” [6]
 
Is there anything lacking in the physical body of Christ? No. Saint Paul is not speaking of the physical body of Christ, but of the Mystical Body of Christ, in which many things could be lacking. For example, we could lack faith, hope, and charity, or in afflictions. Saint Paul, through his suffering, fills what is lacking in sacrifice. The poor widow, through her generosity, fills what is lacking in charity. What can we do to fill up what is lacking in the Mystical Body of Christ? In other words, how can we help the Mystical Body of Christ be more like that of the physical Body of Christ? 
 
Where do the Physical and Mystical Body of Christ unite most perfectly? The most perfect unity lay in Heaven. It is in Heaven that the Church triumphant, the Bride of Christ, will meet their Groom. The Bride of Christ, the Church, receives a foretaste of this glory at the Holy Sacrifice of the Mass at which she receives the physical Body of Christ in the Eucharist. This is the same physical Body that she hopes to embrace for all eternity in Heaven.
 
During the Holy Sacrifice of the Mass, is Christ “re-crucified”? The Mass is the re-presentation of Calvary. It is not the physical Body of Christ that is re-crucified at Mass. Christ suffers not through His physical Body, but rather through His Mystical Body which is forever united to Him in the passion, death, and resurrection.
 
What does Christ do with His Mystical Body on earth? “Our Lord in heaven does the same three things with it that He did with His individual human nature taken from Mary; namely, through its instrumentality, He teaches because He is its Teacher, governs because He is its King, and sanctifies because He is its Priest.
 
He is teacher, but He gave the power to His new Body: ‘As the Father hath sent me, I also send you,” “Going therefore teach ye all nations,” and “He that heareth you, heareth me; and he that despiseth you, despiseth me.’ Certainly, if these words mean anything, they mean that just as He once taught through His visible form as man, so now He continues to teach through His new body, the Church, the truth forever being not the Church’s but His, and therefore infallible.
 
He is King with power in heaven and earth. This office He gave to His new Body: ‘All power is given to me in heaven and in earth.’ My Power I give unto you: ‘Whatsoever thou shalt bind upon earth, it shall be bound also in heaven: and whatsoever thou shalt loose on earth, it shall be loosed also in heaven.’ These words mean that He, as He was formerly King in His individual, physical manhood, so now He is King through His new manhood the Church, the Power being not the Church’s but His, and therefore divine.
 
He is priest, Who came to give His life for the redemption of many. This office He gave to His new body: ‘Baptize them in the name of the Father, and of the Son, and of the Holy Spirit.’ ‘Do this for a commemoration of me.’ ‘Whose sins you shall forgive they are forgiven them; and whose sins you shall retain, they are retained.’ Again, the meaning is unmistakable — as He had sanctified souls and offered His own Body and Blood to His Heavenly Father, so now He was communicating that power to His Mystical Body, the sanctification forever being not the Church’s but His, and therefore divine.”  [7]
 
Search: Priest, Prophet, King

[1] CCC 791
[2] Archbishop Fulton Sheen; The Fullness of Christ
[3] Hardon; Modern Catholic Dictionary; page 366
[4] John 15:11-14
[5] Philippians 2:7-8
[6] Colossians 1:24
[7] This answer is taken from Archbishop Fulton Sheen; The Fullness of Christ