This phrase is simple and very common. You've probably heard your teacher say "time's up" at the end of an exam. Time's up means you have to finish, and you have to stop what you are doing right now. Usually, an authority figure or someone in-charge will say this at the end of a timed activity. You'll often hear someone say "time's up" at the end of a competition. An example is a contest on who can do the most sit ups in 15 minutes. The contestants do as many sit ups as they can, and at the end of 15 minutes, the referee then says "time's up."
You can use time's up when you're playing a game with friends. When one group is still figuring something out, and they're taking too much time, everyone can say, "Alright guys, time's up. You have to make a decision."
A: I don't think I could eat that many hotdogs, period.
B: I know. ..and they have to eat as many as possible in 15 minutes.
A: That guy in the blue shirt is way ahead of everyone. He's eaten 30 hotdogs already.
B: That's ok. Jim's pacing himself. They have 10 minutes before time's up.
A: I hope you're right. He really wants to win this competition.
A: I don't think he's going to make it.
B: No, time's up. Oh well, better luck next time.
timed activity - an activity that has a time limit.
sit up - an exercise where you lie on the floor with your knees bent, then you sit up and lie down repetitively.
figure out - to understand something; to find the answer
way - very much (in this context)
pacing yourself - control your speed (usually so that you can last a long time)
Alright everyone, the next time you're playing a game, and someone's run out of time, make sure you're the one to say "Time's up!"