in more modern systems rad balancing is not too necessary but if you have an older one, or one which has only one thermostat, then you may need too.
woofy- what it is is that the pump may not pump water efficiently through all the rads as their resistance to movement varies. some will be very hot, others stone cold, as the flow takes the path of least resistance. this is most felt in gravity fed or trickle down systems.
To balance them, first bleed all the radiators that need it. turn all the radiators onto full, turn on the heating and have a cup of tea. this allows time for the rads to heat, as well as being refreshing and full of flavanoids. feel the radiators throughout the house. if all are at the same heat level, dont panic, you dont really need to balance any. if some are hot and some are cold, then turn the cocks on the hot radiators down a little, and check to see if the cold ones warm up. continue to do thisuntil the heeat is even. do it one floor at a time. When all are heating evenly removee the knobs from the radiator stopcocks and hide them to stop females "adjusting" them.