It's an episode almost or entirely composed of clips from previous episodes. Usually it has some sort of a framing device - for instance, in an adventure show, it might be the characters taking a 'breather' after a tough encounter and musing on how they got here. Or one character might confront another about a situation that's been brewing, and the clip show is showing bits of that situation leading up to the confrontation.

On an aside, reception to clip shows is an interesting shift. For a long time, one or two were an accepted part of a long-running series - either because it let you make an episode on the cheap using recycled footage, or because in the pre-internet-streaming-on-demand world, it let audiences catch up on what had been happening in episodes they might have missed or seen months ago.

Nowadays, however, they're almost universally viewed negatively, as their reason for existing is absent and they're mostly taken as a sign of poor planning by the creators.