What does it mean when a comic has it issues collected in "Volumes"?

So I am very sorry if my question didnt make a whole lot of sense. When I say "volumes" I dont mean like the hardback and paperback collections they sell at bookstores and such, what Im referring to are old issues of a comic book series divided into volumes. Im more specifically talking about Green Lantern because that is what im currently reading. So I think right now the series is on "Volume 4" but I have no idea what this means? where is the line draw to divide the old comics? I was reading bios on old characters in the back of a GL book and some characters were introduced in "V.3", "V.2" and so forth. So im just wondering how the comics are divided in volumes? Can anyone help?