What does John 6:51 mean?

As the people begin to complain about Jesus' claims to have come from heaven (John 6:38), He gives a deeper explanation of His meaning. In the wilderness, God sent physical bread from heaven to meet a physical need. That manna symbolized Jesus. Manna had to be accepted as a gift or simply walked on. Now, according to Jesus, God has sent the "true bread from heaven" (John 6:32), in the form of a person (John 6:33). This is Christ Himself (John 6:48). Eternal life is only for those who believe in Christ (John 6:40). This explanation came after the people misunderstood the symbolic nature of the "Bread of Life" (John 6:26, 34).

This verse contains the statement which truly derails Jesus' worldly popularity. Prior to this conversation, the people were ready to make Jesus king—by force, if necessary (John 6:15). Showing that His ministry is spiritual, not physical, has taken some of that enthusiasm away (John 6:41). Now Jesus refers to the real role of the Messiah: to suffer and die for the people. The offering which will grant eternal life to those who believe is Jesus' body: His flesh.

This comment contradicts the prevailing Jewish view of Messiah as a warrior-king. Worse, it sounds like cannibalism to those still fixated on material things. As the next passage shows, when Jesus makes this claim, it pushes the crowd from complaining to outright fighting.