In simple terms - just the power of the pump. I.e. the volume of water it can move. This is sometimes measured in terms of 'a head of water' in height.
A 6m head pump is more powerful than a 5m head pump.
depends on size of house & your system.an average of 7 to 10 rads with small bore (15mm pipes @ rads)you will only need a 5m head pump but if you have a lot of rads or microbore (8mm pipes @ rads) you are better off with a stronger pump