Haglund’s prominence is a bony enlargement on the back of the outer heel.
The retrocalcaneal bursa is a fluid-filled sac located at the back of the heel under the Achilles tendon. It contains lubricating fluid that reduces friction between the tendon and bone (see image and Achilles & Heel - Anatomy and Imaging).
When this prominence rubs against shoes or the Achilles, the bursa and tendon become irritated. This often leads to painful inflammation (Achilles tendinitis and retrocalcaneal bursitis), and this is called Haglund's syndrome.
Interesting fact: some people call this condition "cucumber heel" - I don't know why.