Sometimes air becomes trapped in a central heating system, which has the result of the radiator being cold at the top and often warm at the bottom. Removing the air or 'bleeding' the radiator allows it to fill with hot water.
To do this you must first have a special radiator key, these can be brought at your local D.I.Y. shops. You will also need a cloth and a cup to catch the excess water.
Firstly, switch off the central heating system. The bleed valve is the small square nut at the end of the radiator. With the key, gently turn the nut anti clockwise until you hear a hiss, this is the air within the radiator being released. When water starts to come out of the radiator turn the key back clockwise to shut off the valve.
Do not unscrew this valve completely as the plug will come right out.