Glossary
Ace
In one line: One player kills all five enemies in the same round.
What it means
An ace means you alone kill all five enemies in a round. The game announces it to everyone, and it is the best moment of the match for whoever pulls it off.
It is worth knowing why it happens: an ace almost always comes from enemies arriving one at a time. If you can fight one opponent at a time, even a 1v3 becomes three 1v1s. That is why the player who aces often does not have the best aim in the lobby, but picked good spots and good moments to shoot.
An ace is not a goal: the team wins the round. Chasing one at all costs leads to selfish choices, like keeping information to yourself to get the kill.
An example
You are the last defender alive on B, 1v4. The enemies come out of main in pairs: you take two from a tight angle, then move and catch the last two looking for the spike. All five kills in the round are yours: that is an ace.