ABX.HE is a part of the larger derivatives market. In essence, ABX.HE is an index of of subprime credit default swaps. Credit default swaps are a sort of investor's "insurance".
So, for example, if you were a major investor and had invested heavily in subprime mortgages you could buy credit default insurance in case those mortgages collapse.
You, as an investor in that market would be providing the funds that pays those policies - you'd be betting that the mortgage holder's don't default which, to be honest, doesn't sound like the smartest move right now...
I doubt there are many individual investors in that sort of marketplace. This is some serious, heavy duty, "you had better know what you are doing" investing done by major financial investment firms who employ thousands of people to crunch a whole lot of numbers. You aren't gonna get into this with an over-the-counter buy on E-Trade.
If you really want to get into this sort of thing you'd need to talk to an investment house that deals with Institutional investors and you should probably plan on bringing a few million with you to get into the game.