A situation in which you win a game easily by destroying the enemy team without much effort, or by making very successful plays. "Ez clap" is commonly seen in Overwatch, although it is relevant in other games.
After they stomped the enemies in the first round of the game, they exclaimed "ez clap" in the all chat.
Contrary to other definitions, the origin has nothing to do with "clapping" your opponents' cheeks or some such. It derives from Twitch chat's BTTV emotes, where typing "EZ Clap" shows a smug Pepe the frog clapping his hands.
Used to express the absolute and complete domination of the opponent. Use of the phrase became widespread and popular because of the realization that the booty go clap clap clap