Since I hurt my ACL, I've been reading a lot about doing closed-kinetic chain exercises after knee injury. What kind of exercises are these anyway?

Closed-kinetic chain exercises are done with the foot or feet planted firmly on the ground or some other surface. This type of exercise is preferred because it helps reproduce normal, everyday movements.

Squatting, stepping, and stair climbing are examples of closed-kinetic chain activities. The exercises are functional but also reduce the strain and shear force on the ACL. In fact, they also decrease the compressive force on the patella (kneecap), too.