Glossary
Hold
In one line: Waiting still with your crosshair on a passage, ready to shoot whoever appears.
What it means
Holding an angle means staying in a spot with your crosshair on one precise point: the end of a corridor, a door, a corner. Whoever appears there has less time to react than you, because you are already set.
Holding well means choosing one thing to watch (not three), keeping your crosshair at head height and knowing when to stop: if nobody has come for a long time, the attack may be elsewhere and you are wasting a player.
An example
On defense you hold the exit of B main from a side angle, crosshair on the edge of the door at head height. The first attacker running out does not see you in time: one shot.