What is it?...it is smelly, messy and IMHO not worth the effort. For co2 to be enhanced effeciently, you really need a sealed grow, with thermostaically controlled exhaust/intakes, and meters, guages 'n tanks.
That being said, I think the recipe is 2 cups of sugar, 1 package of yeast in a gallon of water. Remember, co2 is heavy, so it should be dispersed above your plants.
I concur with Hick.
High Times did the average home growers a great disservice about 15 years ago when it touted CO2 as the "magic bullet".
What is does is speed up veg.
When marijuana (and most plant life on earth) evolved, there was more CO2 in our atmosphere.
I don't feel like getting out my book but I think that back then it was 1600 ppm and to-days level is 300 ppm. So plants evolved to utilize these higher amt's.
To effectively use CO2 augmentation, you need a atmospheric ppm meter and that costs $500.00.
Most home growers would be MUCH better off by optimizing ventilation. Pot grows great with flo-through ventilation. Not a fan inside a room (that just blows hot air around), not just a fan inside blowing out an open door (not much stealth there). For good ventilation you need to cut 2 holes and put in an intake and exhast fan ($12 each @ home depot).
co2 of some sort will increase bud size considerably. 1gal jug with 1/2gal water, 1cup sugar and a pack of yeast. poke a small hole in the lid and shake frequently while covering the hole with your thumb. co2 will rush out when you move your thumb off the hole. mix up a couple or three if you can. wont hurt a thing, just make your buds fat.
poke a hole in top, insert straw(seal with a piece of gum), and aim at plants
Don't have any experience growing pot with administered CO2, but I have kept many kinds of aquatic plants for several years. There too, the delivery of the CO2 has to be done perfectly and effectively for it to be a real benefit. The only worthwhile CO2 systems for tanks are very expensive.