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.

The Progressive Aerobic Cardiovascular Endurance Run (PACER) is a multistage shuttle run created by Leger and Lambert in 1982. The PACER is designed to measure aerobic capacity, which is characterized by endurance, performance, and fitness.

https://www.nova.edu/projectrise/forms/pacer_manual_42309_jk.pdf

It sounds like something we did (in the US) in gym class, but I’ve never heard it called that before.

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.