Ligaments connect one bone to another bone. Therefore, they allow a joint to form, because joints are where two or more bones connect.
Take your knee joint for instance (***see attached pic***). The knee joint is formed by the connection of the femur (your thigh bone), the tibia (your shin bone), and the fibula (the other long bone in your lower leg). In order for all of these bones to connect there are many ligaments in the knee joint that keep the bones connected and in place. A well known example of one of these ligaments in the knee joint is the ACL (anterior crucate ligament), which is commonly torn in sports, namely football. Most people have heard of this ligament because it receives a lot of media attention since tearing it can greatly alter or even end professional athletes' careers.
