The ACL is the anterior cruciate ligament, located in the knee, helping to stabilize the tibia (bone of the lower leg).
And it is stress on this knee joint - particularly direct impact on the knee from the front or the outside -- which causes injury to this ligament.
In particular, females that participate in gymnastics, soccer, and basketball have much higher rates than average for ACL tears just because of the types of movements in those sports.
The typical scenario is a person who is running -- then plants their foot and suddenly slows down to change direction, or does the same and tries to pivot on their leg -- and puts too much stress on the knee while it is outstreched and attempting to turn.
Dribbling a soccer ball and "cutting" to the left or right with a sudden movement is a typical scenario for injury.
Also direct injury - such as a player tackling you from the outside of the knee, causing the knee to buckle inwards is another mechanism for injury (less common though, as most ACL tears come from non-contact injuries)