What does John 6:59 mean?

The previous day, Jesus performed His most public miracle, which is also the only one recorded in all four Gospels: the feeding of thousands (John 6:1–14). In the morning, the crowds travelled across the narrow Sea of Galilee and found Jesus in Capernaum. The earlier portion of this dialogue probably occurred in the streets of Capernaum near the harbor (John 6:24). During this conversation, however, Jesus has repeated several points (John 6:35, 48, 51). The crowd has also gone from seeking Jesus (John 6:24), to complaining about His teachings (John 6:41), to outright fighting (John 6:52).

This verse specifically refers to the synagogue in Capernaum. At some point during the conversation, Jesus presumably moved from the open streets (John 6:25) to the synagogue. This would allow easier back-and-forth between various people. Details such as this suggest that many conversations recorded in Scripture are essential summaries, not necessarily word-for-word transcripts. The idea that the Bible often records the "true voice," rather than every literal word, is known as ipsissima vox.