As some others stated above, "bling bling" normally refers to big, expensive, shiny jewelry. Normally the platinum chains etc that just about all the rappers wear. They just say BLING BLING because it's so shiny and really catches your eyes.

But.. in slang terms, it's not only limited to jewelry, we associate anything that involves a lot of money etc with it. For example, if someone goes from being very poor to being extremely rich, you might say "oh he's bling blingin' now", but that doesn't necessarily mean that he's buying platinum chains (he might not even own any ;) .... just that he's all big and rich now.

Or if someone has a nice car, you might be like that's BLING BLING man.

Hope that makes it a little easier for you all to understand. If anything is unclear, just ask. :)

Peace.

Shoaib