The shoulder joint consists of a humerus head (ball) and scapula (the shoulder blade). This joint is having maximum movement. The muscles, ligaments, and tendons hold this joint and provide stabilization, both during rest and during making a movement.
When your shoulder joint starts separating or the ball of the shoulder is going away from its socket because the surrounding structures are not able to hold the shoulder bones in their place is shoulder instability. Or your can say your joint become loose. When your joint slides out partially or for few seconds it is known as subluxation, and the separation of joint bones is severe or complete it is known as dislocation.
Your shoulder instability also affects the surrounding structures of shoulder joint like damage to the muscle, tendon, ligament, or cartilage.