Grounding, sometimes called "earthing," is simply the idea of reconnecting your body to the earth's natural electrical charge. It's something people did without thinking for most of history, by walking barefoot on grass, sand, or soil, before modern floors and shoes got in the way.
A grounding sheet brings that idea indoors. A conductive thread is woven right into the fabric, and a cord plugs into the grounding port of a standard wall outlet. So while you sleep, your body may be gently connected to that same natural charge. No batteries, nothing to power on.
Many people say they feel calmer and sleep more soundly after switching to a grounded sheet, though results vary from person to person.