What does it mean when my shoulder pain goes away when raising arm above head?

Rotator cuff : This is usually a sign of rotator cuff tendon impinged at the shoulder.

Ss muscle tendonitis: The Supraspinatus, Ss, muscle of the superior shoulder region functions to lift the shoulder overhead. An injured Ss muscle has relaxation & relief of pain in arm abduction & pain arousal in adduction, as the arm weighs about 20#, & exquisite pain at the sudden drop of a "dead arm"; impinging a previously traumatized Supraspinatus tendon, which inserts on the rotator cuff.