It means kilowatt. This is the unit of measurement Electric Vehicle chargers are sized by. The higher the kW, the quicker you car will charge.
A 7.2kW charger will charge your Vehicle twice as quickly as a 3.6kW one. The onboard computer built into cars is clever, and will restrict your charger's output if your vehicle doesn't support that level of power output. So you should always buy the highest kW option where possible.
Despite the varying kW output, often the chargers themselves are very similar. Where this is the case, BOXT will always simply sell you the best model, unlike some competitors who mark up the more powerful versions.