This booster pump is needed for the Stealth 100 or the Stealth 200 RO water filtration units when you can not maintain a constant 65 psi of water pressure.
This might be a problem if you are on a water well, as the minimum pressure needed is about 40 psi. The filter will still work at pressures of 40 to 50, but your total GPH (gallons per day) will be less than the rated output. For instance, if you only have 45 to 50 psi you might only get 100 GPD from the Stealth 200, where with the booster pump you can count on 200 GPD.
It is usually better to buy larger than you think you need, if you can.
Unit comes with a 3/8″ quick connect fitting, a high pressure switch, and a low voltage transformer for safety.