Bursa

Wherever tendons or muscles glide and overcome a bone edge or a bone protrusion, there is a bursa. A bursa contains in its interior a clear, viscous and thread-pulling liquid (Synovia).

Imagine a balloon half filled with water and knotted. It's on the table and you put your hand on it. Now you can move your hand back and forth a certain distance, whereby the balloon with its support does not slip, but remains in place. They create a flexing motion. That's how a bursa works. Usually it has grown together on one side with the adjacent tissue. The ″Walk″ page is designed to ensure trouble-free movement of the tendons and prevents reduced blood circulation due to the pressure exerted.

https://de.wikipedia.org/wiki/Schleimbeutel _{(Wikipedia CC-by-sa-3.0)}