Practice ends. Everyone says the same thing to everyone else, and keeps saying it — walking out of the gym, passing in the corridor, closing a chat window. Subtitles render it as “good work”, “thanks”, “see you”, or quietly give up and leave it out.
It isn't a greeting, it's an acknowledgement
つかれる means to get tired. おつかれさま is that, wrapped in polite clothing on both ends: “you must be tired.” It says I can see you have been doing something. That's why it fits so many slots at once — arriving, leaving, thanking, congratulating — because all of them are moments where noticing someone's effort is the right move.
Two forms, one rule of thumb: おつかれさまです during, おつかれさまでした after. Between friends it shortens to おつかれ, which is where anime clubs live. And it works upward, downward and sideways — juniors say it to seniors and back again.
The one that looks similar and isn't
ごくろうさま also praises effort, but it only travels downward — a boss to staff, a client to a delivery driver. Aimed upward it is startlingly rude, because it takes the position of the person handing out the assessment. If you remember one thing here: never say ごくろうさま to a teacher, a client or a せんぱい. おつかれさま is always safe.
おつかれさまでした！
Good work today!
おつかれさま。
You too.
Can you use it?
Constantly, and you should — it's the single most useful phrase in a Japanese workplace or club. It also solves a problem beginners have: what to say when you pass a colleague and こんにちは feels too formal but silence feels rude. This is the answer.