It is supposed to boost immune system, reduce recovery time, blah blah blah...It's the most abundant free form amino acid in your blood stream, but it doesn't do that much directly. IIRC, it is coverted by the body into various proteins as the body needs it.
Personally, I think it's shit. I used it extensively for years before I ever juiced. I tried a lot of supps to gain an edge in the gym. Most are shit, IMHO, and glutamine of any brand (I tried several) is not worth the money. Most whey protein is loaded with glutamine, anyways, so just make sure you're eating 2 grams of protein for each pound of your bodyweight. You don't need extra glutamine supplementation.