Bling is a slang term for fancy, expensive jewelry and accessories. You might aim to impress on the first day of school by wearing a lot of bling.
There's an implication in the very term bling that not only do you have flashy chains and watches, but you want to show them off. The word itself comes from the idea that light glinting off gold or diamonds would make a bling-like sound if you could hear it. Both bling and bling-bling are US rap slang which gained mainstream popularity around the mid-1990s.