Your regulator is one of the main pieces of scuba equipment that allows you to breathe while underwater. The second stage contains the mouthpiece and purge valve; the purpose of the second stage is to deliver the air into the diver's mouth and to remove any waste (air or other objects) as needed.
Purge Button/Exhaust Valve
The 2nd stage's exhaust valve is a one-way valve that allows the diver's exhaled air to leave the regulator. Basically, it is where the bubbles exit the mouthpiece.
The purge button allows the diver to force air from his/her tank through the regulator to clear any water that may have inadvertently entered the 2nd stage. This button should be fairly simple to manipulate, even when wearing thick dive gloves. It should also only require the pressure from one finger to successfully perform the purge.
Your 2nd stage's mouthpiece is a more crucial piece of equipment than most people believe. A mouthpiece should be comfortable in your mouth; if it feels too big or too small, use a different size mouthpiece.