A 'Bursa' is basically a small pouch of a gel-like substance that is located between a bone and muscle attachment. Its role is to act as a lubricant and decrease the friction on the muscle's tendon as it repetitively moves and rubbs over a bone's edge.
'Bursitis' is the name of a condition in which the bursa becomes inflamed, swollen and occupies more space around a joint then it should. Typically this occurs due to overuse and excessive muscle tightness causing compression on the bursa. Bursitis is more prevalent surrounding your shoulder joint and your hip joint.
Physiotherapy can help to take the load off the bursa by releasing the tension compressing it, and can treat the true cause of the symptoms by addressing poor shoulder girdle posture to ensure that it does not become a recurrent injury.