What is a "light carbon isotope?"?

I'm in a Geology course, wk'g on a study gd. for an exam, and I need evidence for the Snowball Earth Hyp. I get all of it except for the "light carbon isotopes" part. I actually understand glacial striations, etc. Could someone tell me what light carbon isotopes are- in "normal" person language?!! (I'm horrible at science terms- so, it needs to be understandable!)