Glossary
Trade
In one line: Killing the enemy who just killed your teammate, right away, so the round stays even.
What it means
A trade is an exchange: your teammate dies, but you kill whoever killed them within a second or two. The numbers stay even (4v4 instead of 4v5) and the enemy often had to reveal their position to get the kill.
Trading is the foundation of teamplay: you move in pairs, at a distance that lets you respond (spacing). A player who dies where nobody can trade them has handed the enemy a free kill.
An example
Your teammate goes in first and gets killed by the defender in the corner. You are right behind, see where they are and kill them before they can fall back. The round stays even, and the defense now has one fewer player on that site.