It is the cranial cruciate ligament that can be torn (shown with the arrow in the image). It is the same as the ACL in humans! But as dogs have an anatomically different knee (stifle) to us, if this ligament is torn, even just partially, their femur and tibia can slide away from each other (right hand side of top of image), causing joint instability and pain! This rupture can occur traumatically, aka sudden lameness after an incident, or gradually, such as a gradual onset lameness and change in posture. And once one has gone it is very common for the other stifle to have the issue also. It does have a genetic predisposition to it!
Feeding joint supplements is a good idea in general, however these ligaments don’t sit in the joint capsule, as as a ligament the joint supplements won’t influence these ligaments, BUT if stifle health is better in general, that could support the joint and prevent issues. Although there is a lack of research on this.
Best ways to prevent an issue is keeping weight down! Body condition score (see my previous post on this), and to prevent excessive exercise when young, and prevent activities like “fetch” where you launch a ball and the dog zooms, twists and jumps! Hydrotherapy and regular physio can help prevent this issue. If you know your dogs breed line has had this issue before that is definitely the best thing to do to find a good therapist (and get insurance 
The ligament is inside the joint, between the tibia and femur, and once gone it’s gone! Surgery such as TPLO (bottom left of image) or TTA (bottom right of image) doesn’t repair it but changes the anatomy of the joint for biomechanical stability. Physio will be needed afterwards to re-educate gait and improve muscle mass.
Partial rupture can be treated conservatively with physio and hydrotherapy. However the ligament will never fully heal and always be at risk of fully rupturing.
