I would like to know in John 6:53-56 what does Jesus mean by "eat my flesh and drink my blood"? What do flesh and blood represent here?
The statement is found in a discourse recorded in John 6. In the context, Jesus had feed over 5,000 people miraculously and this created a crowd of people wanting Jesus to repeat this miracle. "Jesus answered them and said, "Most assuredly, I say to you, you seek Me, not because you saw the signs, but because you ate of the loaves and were filled. Do not labor for the food which perishes, but for the food which endures to everlasting life, which the Son of Man will give you, because God the Father has set His seal on Him."" (John 6:26-27). This did not detour the people. As the conversation continues they attempt at every turn to argue that Jesus should provide them with free food.
In response, Jesus answers that they are looking for the wrong thing. They are only looking for physical food which will temporarily fill their stomachs. They should be looking for spiritual food which will give them eternal life. "Then they said to Him, "Lord, give us this bread always." And Jesus said to them, "I am the bread of life. He who comes to Me shall never hunger, and he who believes in Me shall never thirst. But I said to you that you have seen Me and yet do not believe. ... And this is the will of Him who sent Me, that everyone who sees the Son and believes in Him may have everlasting life; and I will raise him up at the last day"" (John 6:34-36, 40). Notice how partaking of bread Jesus is offering is equated to believing in him.
Jesus stated, ""I am the living bread which came down from heaven. If anyone eats of this bread, he will live forever; and the bread that I shall give is My flesh, which I shall give for the life of the world." The Jews therefore quarreled among themselves, saying, "How can this Man give us His flesh to eat?" Then Jesus said to them, "Most assuredly, I say to you, unless you eat the flesh of the Son of Man and drink His blood, you have no life in you. Whoever eats My flesh and drinks My blood has eternal life, and I will raise him up at the last day. For My flesh is food indeed, and My blood is drink indeed. He who eats My flesh and drinks My blood abides in Me, and I in him. As the living Father sent Me, and I live because of the Father, so he who feeds on Me will live because of Me. This is the bread which came down from heaven--not as your fathers ate the manna, and are dead. He who eats this bread will live forever."" (John 6:51-58). The theme hasn't changed. The bread is Jesus' sacrifice for the sins of the world. Unless a person "partakes" through faith in Jesus' death, he cannot have eternal life.
Now add to this what Paul later taught. "Or do you not know that as many of us as were baptized into Christ Jesus were baptized into His death? Therefore we were buried with Him through baptism into death, that just as Christ was raised from the dead by the glory of the Father, even so we also should walk in newness of life. For if we have been united together in the likeness of His death, certainly we also shall be in the likeness of His resurrection, knowing this, that our old man was crucified with Him, that the body of sin might be done away with, that we should no longer be slaves of sin. For he who has died has been freed from sin. Now if we died with Christ, we believe that we shall also live with Him, knowing that Christ, having been raised from the dead, dies no more. Death no longer has dominion over Him. For the death that He died, He died to sin once for all; but the life that He lives, He lives to God. Likewise you also, reckon yourselves to be dead indeed to sin, but alive to God in Christ Jesus our Lord" (Romans 6:3-11). Notice that the same themes of death and life are being stated, though worded in a different way. Eternal life is gained by uniting with Christ in his death. That participation in his death comes through baptism.
Likewise, another allusion is being made to something else that memorializes Jesus' death for our sins. "The cup of blessing which we bless, is it not the communion of the blood of Christ? The bread which we break, is it not the communion of the body of Christ? For we, though many, are one bread and one body; for we all partake of that one bread" (I Corinthians 10:16-17). As Christians partake of the Lord's Supper, we share in the sacrifice of our Lord and pay tribute to him by recalling his death. "For I received from the Lord that which I also delivered to you: that the Lord Jesus on the same night in which He was betrayed took bread; and when He had given thanks, He broke it and said, "Take, eat; this is My body which is broken for you; do this in remembrance of Me." In the same manner He also took the cup after supper, saying, "This cup is the new covenant in My blood. This do, as often as you drink it, in remembrance of Me." For as often as you eat this bread and drink this cup, you proclaim the Lord's death till He comes" (I Corinthians 11:23-26). Like baptism, the power of the Lord's Supper comes in what it symbolizes to the Christian. Baptism is not physical death, but a representation of spiritual death to sin. The Lord's Supper is not the physical eating of Christ, but a symbolic memorial to keep fresh in the Christian's mind that he is saved because his King gave his life to free him from sin. It reminds us that we have willingly entered into a covenant relationship with Christ to do his will and not our own.
So to answer your question, the flesh and blood represent Jesus' death upon the cross for the sins of mankind.