It’s a test that makes you run laps until you no longer meet the threshold. [This specific variation] (https://youtu.be/Y82jDHRrswc) has been used in a lot of schools, so it’s become a bit famous.

I think outside the US it’s usually referred to a “shuttle run test”. If you’ve got any memories of running up and down the gym trying to race the beeps on a CD, it’s that.