What does hypertensive signal within the acl mean? what happens to the knee when you have a piece of carr floating around within the knee joint?

Difficult to say: with limited info and without looking at the images, but it may mean that there is an ACL sprain/partial tear. Cartilage "floating" in the knee is termed a loose body and can cause a variety of symptoms including pain, locking, clicking, and degenerative arthritis.

Tear: Usually when you see signal changes with the ACL it indicates at least some degree of tear, issue is whether it is partial or complete. A loose body can potentially damage the articular cartilage of the knee. I would not hesitate to discuss your specific case with your doctor.