CO2 is used to produce extra carbon dioxide into the tank to increase healthy fast growth of plants, its very beneficial for difficult to grow plants, and too much in the tank can effect your tank stability, but most are default ed to add just enough.
mine cost £8 but was retailed at £14. the pack comes with
1 chamber
1 pipe
1 ladder
2 stabalizer
2 yeast stuff
all you need to do is add sugar and tepid water to get it started, make sure that is compatible with your size tank before you buy it. most people are now using homemade ingrediants as refill, i cant remember where on the forum i saw the thread but i think it was on the pinned section of plants section