Quoted for Combi boilers, a flow rate represents how many litres of hot water comes out of the tap in a minute. Your flow rate is determined by the amount (and pressure) of water that is entering your property from the water main and the ability of the boiler to heat that water to the temperature you have set. The more powerful a boiler is (represented in kW), the faster the boiler will be able to heat the water coming into your home.
However it is important to recognise a high kW output will not always increase your flow rate. If you have 10 litres per minute coming into your property, you will only be able to receive 10 litres per min of hot water. For example having a boiler that has a maximum flow rate of 14 litres per min will not increase the amount of hot water you receive, if you only have 10 litres per min coming into your boiler.