This is one of the most important things to a marijuana plant with big buds. Co2 is a chemical in the air that all plants take in and turn it in to oxygen. Us humans being mamals breathe out Co2 every breath we take. This chemical is so improtant because a plant is made up of carbon that it takes out of the Co2. Without this chemical plants would not get any new growth on them so you would have a small pathetic looking plant.
If you are growing indoors you have the ability to provide you plant with 10x more Co2 then the plant could get outdoors. This is good because you plant will grow much bigger alot faster. Which i think is what you want. Now there are tons of ways of supplying your plant with Co2. First you can buy a Co2 canister at many local hardwear stores. But you can use the gift God gave us and use the Co2 we breathe out. No your not going to sit there all day breathing on your plant. What i did was i got a ballon hooked it up it a hose melted the hose close and poked a little hole in it. This will allow a small leak out to supply your plant with the Co2 it needs to grow. I will have a pic of how i setup my Co2 supply.
If your planting indoors you dont have to supply it with Co2 because humans breathe out alot of Co2 so there will be enough with out you making another supply. One thing that you will need this way is air circulation so that you dont have already used Co2. If you have a little bathroom exhust fan you could use that. These can be purchased in local hardwear stores of about 10-15 dollars. If you planting outside you cant do anything about it. The only thing you could do is build a little green house but then you would attract attention which is what you dont want.