Glossary
Choke
In one line: A narrow passage players are forced through, one or two at a time.
What it means
A choke (or chokepoint) is a bottleneck: a door, a corridor, the exit of a main. Attackers must go through it to reach a site; defenders know this and keep it under fire.
That is why chokes are where most utility gets used: smokes to cover whoever crosses, flashes to blind whoever is watching, mollies to stop anyone coming through. Crossing a choke with no utility against a ready defense usually means losing your first player.
An example
The entrance to a site is a corridor two players wide. Defenders watch it from two different angles. Your team smokes one angle and throws a flash over the choke: three of you cross while they cannot see.