A pressure sore is a localised injury to the skin or underlying tissue. It's caused by unrelieved pressure, friction and/or shear (when the upper and lower layers of tissue move in opposite directions).
Pressure injuries usually form in areas where the skin is close to the bone, such as the heels, elbows, tail-bone and back of the head.
Older people are more susceptible to pressure injuries because their skin is thinner and more delicate. Reduced mobility or poor health is also a significant risk factor.
Prevention and management of pressure injuries is important as they can lead to serious health complications. I can help by:

(The above illustration is taken from "Preventing Pressure Ulcers: An information booklet for patients" by the Victorian Quality Council.)
See the Victorian Department of Health & Human Services' Better Health website for more information on pressure sores.