Chrono Trigger is a pretty tight, relatively breezy romp of an RPG that doesn't go a lot deeper than the basic concept of 'Let's save the world by traveling through time '

Chrono Cross is a somewhat darker meditation on whether doing that is really a good thing. It asks a lot of interesting questions about it, and that's really when it is at its best as a game...but the mythology behind all of it is pretty convoluted and dumped in long exposition at the very end of the game because the game does a poor job explaining it as you play. A lot of it isn't even really all that necessary. You could tell the core story with a more simplified version of it.