My sister has compartment syndrome. what does this mean?

Leg swelling/pain: The muscles of the extremities are compartmentalized or occupy spaces of a finite volume anything anything which increases the pressure within these compartments (swelling of muscle, bleeding, persistent external compression, a mass, etc) can impair blood flow into the compartment. This is experienced as pain as the muscle is deprived of oxygen.

Can be an emergency: Compartment syndrome occurs when there is swelling of the muscles in an extremity. The muscles are surrounded by fascia, which can only expand so much. When the swelling is reaches this point you can start to have muscle and worse case nerve damage. This is usually seen with an injury. Emergent surgery is needed you can also develop exercise induced compartment syndrome. This is not an emergency.