Polyurethane is a kind of plastic. The sense you need here is the one in a paint shop: people use it to make paints and varnish — the tough, often shiny coat that goes on wood, floors, and sometimes cars or boats. It dries into a hard skin that water and scratches do not get through easily. Though actually the same family of plastic turns up in a lot of other places too (soft foam, some fake-leather bags), a tin labelled polyurethane, or PU, is usually this coating. It is a technical word. In ordinary talk people just say the varnish, the clear coat, or PU paint. You almost never need it as a verb in school English, though a factory may say they polyurethane a floor, meaning they paint this stuff on.