This hip-hop slang term refers to one's display of flashy possessions to announce his/her financial success. Interestingly, the word "bling" is a perfect example of onomatopoeia as it signifies the sparkling sound shiny things make when light glares off of them, esp. on cartoons.
Prime examples of bling include gold watches with lots of diamonds, platinum necklaces, diamond rings, gold/platinum teeth, really nice rims, and a really expensive luxury vehicle. Having 20" rims (or dubs) on your luxury vehicle is a great situation in which we have bling on bling. (bling bling)