A ball valve you can control the flow of, a check valve keeps whatever media that passes through from back flow. If I'm not mistake he's talking about (aqualifter) dosing pump. I have never used this, so I don't know why you will add it... maybe we will be enlightened.
The aqualifter http://www.tomaquarium.com/webfiles/productspages/aqualifter.html (google is our friend) is just a small water pump that is self-priming (can tolerate air in the system). It constantly pumps out the air in that section before it can accumulate and stop the flow.
If you have fish that splash a lot or a lot of aeration, air gets in the system more frequently.
Doesn't knowledge feel good! Kinda tingly all over, huh