In the plumbing trade bleeding means to drain a pipe, tube, or hose of excess air by opening a valve at the end or systematically removing the air by force or suction.To remove the air inside your central heating system that has gotten trapped you need to bleed your radiator. Air in a radiator will rise to the top forming a pocket of air and stopping the hot water from getting to it, when there is too much air inside your system it will prevent it from heating properly.