Glossary
Lurk
In one line: An attacker playing alone, away from the team, to catch defenders from behind or cut off rotations.
What it means
The lurker is the attacker who does not go with the others. While the team hits a site (or pretends to), they stay on the other side of the map: waiting for defenders to rotate so they can catch them from behind, or finding a site left empty.
A lurk only makes sense if it creates a problem for the defense: either they rotate and the lurker punishes it, or they do not and the team attacks against one fewer defender. A lurker who hides all round and does nothing is just one player down for the team.
An example
The team goes A with lots of noise. You stay quiet near B. At 30 seconds you hear a defender running from B towards A: you catch them from behind as they rotate. Now your team on A fights 5v3.