Glossary
Clear
In one line: Checking an angle or area to make sure nobody is there.
What it means
Clearing an angle means looking at it, crosshair ready, before you move past it. A site has lots of places where a defender can hide: you check them one at a time, the most dangerous first, and split the work with your teammates.
The classic mistake is walking in looking straight ahead and taking a shot in the back from a corner nobody checked. Another rule: say out loud what you cleared ("behind box clear"), so nobody checks it twice and nobody forgets it.
An example
Three of you enter a site. You clear the left corner by the wall, a teammate clears behind the boxes, the third watches the defenders' exit. In two seconds the site is checked and nobody looked at the same spot twice.