A car is on a one-dimensional track, positioned between two "mountains". The goal is to drive up the mountain on the right; however, the car's engine is not strong enough to scale the mountain in a single pass. Therefore, the only way to succeed is to drive back and forth to build up momentum.
MountainCar-v0 defines "solving" as getting average reward of -110.0 over 100 consecutive trials.
This problem was first described by Andrew Moore in his PhD thesis [Moore90].