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.