What is "carpet burns"

Usage examples of "carpet burns".

Her face was bruised, she had carpet burns on her knees and a small cut, too minor to require stitches, on her foot, and her ribs were sore, probably from the struggle.

My father favored a chair in the basement, but my mother was apt to lie down anywhere, waking with carpet burns on her face or the pattern of the sofa embossed into the soft flesh of her upper arms.

There were carpet burns on her knees from the struggle as she'd gone down.