Matthew 14, Mark 6, Luke 9 and John 6 all report a version of the same episode: a crowd that had followed Jesus into a remote place had nothing to eat, and from five loaves of bread and two fish, blessed and broken by Jesus, the disciples fed a crowd Matthew and Mark number at five thousand men, not counting women and children. Twelve baskets of broken pieces were reportedly left over afterward, a detail present in every version and read by John as deliberately significant, echoing the twelve tribes or the twelve apostles.
Aside from the Resurrection, this is the only miracle story that appears in all four gospels, a fact New Testament scholars treat as unusual and significant in itself. The Synoptic Gospels (Matthew, Mark and Luke) share a great deal of common material and are not fully independent of one another, but John's gospel is composed separately and by most reckonings draws on different traditions, so its agreement with the Synoptics on this episode, down to the count of loaves and fish, is one of the strongest cases of multiple, largely independent attestation available for any single incident in Jesus's ministry. Later interpreters have also noted that counting only adult men, as all four gospels do, likely undercounts the true number fed once women and children present that day are included, a detail that has drawn its own comment on ancient crowd-counting conventions.
Sources The New Testament, Gospel of MatthewAnonymous with The New Testament, Gospel of MarkAnonymous, The New Testament, Gospel of LukeAnonymous, The New Testament, Gospel of JohnAnonymous