Water-ressure-tanks usually have a flexible-sealed-bladder of air;
when your water-pump is running,
it pumps the water-pressure-tank full of water,
compressing the sealed-air-bladder.
When your pump turns off,
the air-bladder is still compressed applying pressure to the water
which forces a few gallons water out of the tank when you open a faucet.
Water-pressure-tanks ease the start/stop load on your water-pump.