Glossary
Site
In one line: One of the map areas where the attackers can plant the spike: A, B and sometimes C.
What it means
Every map has two or three sites (called A, B and, on some maps, C). They are where the attackers can plant the spike, so they are the heart of every round: the attack tries to take one, the defense tries to hold them all.
Each site has its entrances (main, mid, sometimes others), its angles and its callouts. Knowing a map's sites well is the first step to playing it: where you enter, where a defender hides, where you plant.
An example
On a three-site map the defense cannot put two players on each site. Usually they put two on one site, two on another and one on the third, and rotate based on information.