Booster pumps are used to increase or "boost" liquid pressure in a system.Almost any pump might be called a booster pump.
The term is most frequently used to descripe a pump that is installed at the inlet to another pump to increase the liquid pressure and thereby increase the NPSH available to the other pump, which is frequently called the main pump.
The main pump could  be a centrifugal pump which requires more NPSH that is available from the system, a good  example is a high-pressure multistage pump.
The most critical requirement for a booster pump is low NPSH required, this is usually accomplished by operating the booster pump at lower speed.