A bleed valve is the small metal square or screw located within the surrounding nut, usually found at the top of the radiator corner, either on the left or right hand side.
It can be opened to release trapped air from within your radiators or heated towel rails.
Our guide on How to Bleed a Radiator offers a more detailed insight into bleed valves, and how they can be adjusted to help you make the most of your home heating.