The anterior cruciate ligament (ACL) is one
of the four major ligaments
of the
knee, and resists forward movement of the tibia.
If this is a new injury, treat with RICE -- rest, ice, compression, and elevation. NSAIDs can help with pain and swelling, although, always take NSAIDs with food.
My son destroyed an ACL playing football -- he lives a normal life without one, but uses an orthopaedic knee brace when he runs or plays sports.