What exactly does the last supper mean?

Personally, I think it means to express each other's sincerity and trust. The story in the picture is that when Jesus said that someone betrayed him, the twelve disciples showed various expressions and actions: surprise, anger, curse and nervousness, especially Judas' behavior, which was vividly portrayed. The whole picture, the change of mood, the expression of personality, the tendency of action and the combination of characters are obviously dramatic and conflicting, which is a good metaphor for the struggle between light and darkness, beauty and evil.