Motocross is a motorcycle racing sport. In Motocross, racers take on an outdoor course, which often includes a variety of terrain to navigate and maneuver through.
Motocross is named as such due to the combination of motorcycles and the cross-country style circuits they have to race around.
Unlike MotoGP, people tend to consider Motocross as a more dangerous sport. This is because of the close proximity of the racers and the off-road, and often wild nature of the tracks.
As a result, you can be quite likely to pick up some kind of injury in the sport if you are not careful. The injuries sustained tend to be more severe too, due to the high-speed of the motorcycles and presence of other riders.