Pocket v sac
is a tiny pouch at base of each hair that grows on the body.
If one of these follicles get clogged, a collection of sebaceous material or broken down hair plugs can form, and when surrounded by a wall, it becomes a cyst. Often these grow under the skin (subcutaneous), and with hot compresses may resorb on their own. Sometimes they don't and may have to be drained or removed by the doc.