'Bleeding' the radiators means letting out any trapped air, by use of a special key which opens the valve. You have to do it for each radiator, and then the air locks can't prevent the hot water circulating.
A power flush involves pumping the whole system through to remove any sludge inside the rads and pipes. (The plumbers put a product called Fernox into the system, which prevents internal corrosion, but it isn't 100% effective).
Imagine my husband's amazement when he first came to UK from Ivory Coast. He'd never in his life seen a radiator, or a boiler, or a hot tap (or any tap in fact) or a bath. Plumbing was a complete mystery to him.