When you step on the brakes you activate the pads that touch your tyres and slow you down with friction. If you are driving through rain a small layer of water will develop inbetween the tyre and break pad. This will cause your vehicle to hydroplane and loose control of your vehicle. If you apply the vehicles brakes slowly this reduces the chances of the vehicle loosing control due to the hydro planing not being present. It is also advisable to let go of the gas in heavy rain rather than braking. Hope this helps!
