Yes, you can use water straight out of the tap. However, tap water may leave mineral deposits inside the tank, especially if you have hard water in your area. When mineral deposits build up, it limits the amount of steam coming out of the unit. SteamFast recommends using distilled or de-mineralized water in order to avoid this problem.
If you need to clean mineral deposits out of your unit, you can use a solution of vinegar and water, or (for specific units) the lemon acid packet included with your purchase. Please refer to your user manual for unit-specific instructions.