Glossary
Win condition
In one line: The thing that wins you the round if you achieve it: the minimum plan you need to pull off.
What it means
The win condition is the answer to "how do we win this round?". On attack it might be "plant on B with three players alive"; on defense "hold mid until our teammates rotate"; in a clutch "run the clock and catch whoever defuses".
Thinking in win conditions helps you choose: every action either moves you closer or further away. A kill that does not serve the win condition (chasing an enemy while the spike is about to blow, for example) is often a mistake.
An example
You are 2v2 with the spike down and 25 seconds left. Your win condition is not killing both defenders: it is stopping seven seconds of defusing. So you sit on two angles watching the spike instead of hunting them.