Socarrat.

Saveur on the art of paella ...

There are a few old-fashioned paella-making basics that Valencian cooks don't mess with. All paellas start with a sofrito, or flavor base, of chopped vegetables cooked in oil—typically garlic and tomatoes, and sometimes onions and Spanish red peppers called ñoras. The longer the sofrito cooks, the darker and more intensely flavored the paella will be. Also indisputable is this: once you've stirred the rice with the cooked sofrito and the stock, you leave it alone, uncovered. When the rice is cooked through, after 20 minutes or so, some cooks blast the heat to create a flavorful crust, called socarrat on the bottom of the pan.