Glossary
Stall
In one line: Wasting the enemy's time with utility or positioning, without necessarily fighting.
What it means
Stalling means buying time: on defense, slowing the attack down so teammates can arrive; in a clutch, letting the clock run. You do it with abilities that block or scare (walls, smokes, mollies, traps), with fake noises or by showing yourself for a moment somewhere the enemy then has to check.
A stall does not win the round by itself: it helps if someone uses the time you bought. Burning all your utility on a stall when nobody is coming is just waste.
An example
You are alone on B and four attackers are coming. You molly the entrance and smoke the choke: the four wait another five seconds before going in. In those five seconds your teammates arrive from mid.