A sheet or bed sheet was a piece of fabric that covered a bed.
In 2373, in an attempt to woo Kathryn Janeway into having his child, Q replaced the Starfleet issue sheets on her bed with red satin ones, claiming that Starfleet issue sheets gave him a rash. (VOY: "The Q and the Grey")
Thaddeus Troi-Riker's bed had plaid sheets on it. As of 2399, they were still on the bed, just as they had been when he died. (PIC: "Nepenthe")