Your Hoover dryer collects the water removed from the wet clothes, in a tank mounted in the door of the machine. Once the tank is 2/3 full it should be emptied. Do this by opening the door and carefully lift the tank out, it may be heavy so take care when lifting it. Tip the water out of the draining hole in the top of the door down the sink.
It depends on the frequency of use or the type of garments being washed as to how often the tank will need to be emptied out. It can also depend on the size of the spin from your washing machine, high spin speed cycles will remove more during the washing cycle than those with lower spin speeds or if the gentle wash cycles have been used to wash the clothes.