For various reasons, hot water heating systems can accumulate pockets of air in the top of the radiators. When there is too much air, the water doesn't flow any more and the heat slows down and stops. For that reason most people have to "bleed" the radiators regularly: open a little valve in the top of each radiator and let the air out. This is a long and sometimes hot job.
Manual bleeding
If you have to bleed a lot of radiators start on the lowest floor of the house and the radiator closest to the boiler, then work your way around that floor, then up to the next floor. Since air bubbles move away from the boiler and float up in the system, that way you will be more effective in getting rid of water than working from the top down.
Automatic bleeding
Maid-O-Mist is one example of a commonly available automatic radiator bleeder. It is a chamber that is screwed on in place of the traditional bleeder valve. Inside the chamber is a float valve. If it is full of water, the escape route is blocked off. If it fills with air, the float drops down and the air escapes out the valve.
These devices are not flawless and, in a case where any water spill at all would be a disaster, you may want to attach a drain tube to them. But they certainly do help to keep your radiators hot and quiet.
Continuous air scrubber
For the total solution you need an "air scrubber" or "air eliminator" called a Spiro Vent. This is a system worth looking at carefully although only available through registered plumbers.