Essentially, multiple players play the role of crewmates trying to complete tasks. But a few are chosen to be Imposters, tasked with killing, sabotaging and above all... not getting caught. This Among Us meme shows what that feels like.
The main way of identifying Imposters is by catching someone acting suspicious (or "sus," for short). The crewmate in this Among Us meme... totally normal behavior. Not an Imposter, just bad at remembering to lock the door.