Glossary
Post-plant
In one line: The part of the round after the spike is planted: the attackers must stop the defuse.
What it means
After the plant the roles flip: now the defense must come in (to defuse) and the attack must hold. The attackers have a big advantage: time. They do not have to kill everyone, only prevent seven seconds of defusing before detonation.
A good post-plant puts players in spots that watch the spike from different angles, keeps utility to stop the defuse (denial) and does not go hunting for defenders. The most common mistake is pushing out to fight when waiting was enough.
An example
Spike planted on B, 3v3. Instead of moving towards the defenders, you set up on three angles that watch the spike and wait. Whoever tries to defuse is shot by at least two of you.