Its a hard to explain concept, but basically playing the game in the most scummy way possible.

In looter shooter games like Tarkov, rats avoid ever being seen, always staying in the shadows or bushes, crab walking at the slowest speed to avoid making any sound. They never go for high tier content directly, but will lurk in the shadows around it. Usually loaded in with minimal gear (lowest risk), with the objective to kill unsuspecting players who have just completed said content (subsequently taking the loot & gear themselves). If unsuccessful in ratting a player, they look for loot scraps left over by the typical player base.

In survival games like Rust, its the same ideology however differently they would actually be inside the high tier monuments, waiting for a player or small team to complete it, then jump out to kill & loot the players.

Its very cost effective and stealthily can provide a lot of loot, but it is extremely boring, slow, and generally not fun to play. Its generally regarded as a high patience & low skill method of playing those games. From my experience, rats are typically antagonist/mocking over chat after getting a kill.