After the boiler this is probably the most important component.
Simply put the pump circulates the hot central heating water around the system. They deliver hot water to your radiators and back to the boiler again to be reheated and the process repeats.
Old pumps had 3 speeds and were manually set to achieve the correct flow rate. Modern pumps now are ErP (energy-related product) set by the European Union, designed to improve the efficiency and performance of heating and how water products. The difference is that an ErP pump has a variable speed and adjust automatically to the demand in turn saving energy.