It's the pistils of a certain species of crocus.
You've smelled it as part of the seasonings if you've ever had Paella or Arancine, or certain risottos. Also a really good Biryani will be made with a LOT of saffron.
It is a classic spice for rice dishes that are considered special-occasion food, and dyes the rice yellow as it gives that extra rich but delicate aroma.