You can't short real estate easily because it's a physical thing. Subprime was bonds that were secured by mortgage loans on real estate. You can short bonds but it's not easy because you have to find some to borrow. But then someone invented credit default swaps for subprime bonds. CDS are derivatives insuring bonds from default. They are similar to futures by being a 2 sided contract where cash flows between the two parties and the winner pays the loser the difference. One side is long and the other short. In subprime they took it a step further and made packages of these CDS contracts so you could go short a ton of subprime all at once. These instruments coming out coincided with the top of the real estate market in 2005 and their peak popularity was at the peak of the subprime boom in 2007.