What is pressure uncler?

I'm going to assume you mean a pressure ulcer. This is essentially a blister that forms in the surface and deep tissues of skin and muscle, and results from lying/staying in one position constantly. Bony areas of the body which are constantly in contact with a surface will start to erode tissues due to blood flow being cut off. For example, someone on bed rest, lying on their back, that does not change positions may form pressure ulcers on their heels, calves, bottom, shoulders and the back of their head.