Meals included flat bread, which those eating would dip into a shared bowl. Jesus gave a piece to Judas (compare Ruth 2:14). The disciples might have thought Jesus was honoring him.
They did not understand what was happening. They even thought that when Judas left, it was to fulfill an official duty for the feast (John 13:29). However, Jesus was signaling to Judas that he knew Judas's plan and was not surprised. The knowledge Jesus has is deep and complete (see 1:48; 2:25).