Never mind I found my own answer @ http://www.hydroponics.net/learn/faq.asp
Why does an ebb and flow tray system have two fittings?
First of all the common names of those two fittings are a
fill/drain fitting and an overflow fitting. The fill/drain fitting is a fitting that is attached to the pump and serves as the inlet for the water filling the tray and also serves as the outlet for the water to drain back into the reservoir. The overflow fitting consists of a number of different plumbing parts and acts as a watering level regulator. The concept is quite simple... you have a tray that is suspended somehow, or sitting on top of, a reservoir. Now the pump turns on and begins pumping water into the tray and the pump will stay on as long as the grower has determined the watering cycle should be (usually 30 min.). As the water level rises in the tray it reaches the top of the overflow fitting which then allows the water to spill back down into the reservoir. So you have now filled the tray with water and you are maintaining that watering level by means of the overflow fitting, which does not allow the water level to rise any higher. Now when the pump is turned off the water begins draining by means of gravity back down through the fill/drain fitting, and through the pump, into the reservoir. The height of the overflow is decided by the containers or growing media being used.
You only want the water level to rise and submerge about 50% to 75% of the medium or containers. This is to keep the medium or containers from floating and/or tipping over. 
Hey, can you + rep yourself??? LOL!
