If you attend a real one, you get to see a naked woman. If you go to a really real one, you get to eat some wafers containing semen. ;D
Aside from that, Takamba pretty well distilled the meaning of "mass". Some people find it very moving but I've never seen the draw. It is a ritual so it's not really possible to tell you what it "is". If you're interested, you should try to track down a local EGC body, attend one, and find out what it "does" for you. Or, if that's not possible, do a search on vimeo for the official video version from Seattle. It won't provide you with the experience of being in the ritual space but, if your imagination is good, it will give you the framework.
Why Gnostic? My understanding is that the original version (pre-Crowley) was an adaptation of a similar ritual cribbed from l'Église Gnostique de France. The whole Gnostic revival of the late 19th century was an attempt at restoring the direct interaction of individual to deity that the Roman orthodoxy had suppressed about 1500 years earlier. So this ritual is intended to be a dramatically moving representation of the direct interaction of the aspirant with god-head. I think.
Go in all ways contrary to the world.