The olympic truce is an idea that during the olympic games there should be no wars between nations. So during the games, all wars between nations would be put on a truce and the nations would participate in the events together, continuing their war after the games. That is because it varies from source to source, and in general it was connected to divine rituals and things like that, which were not very specific. This was done way back in ancient times so it's not known certainly.
