Sichuan pepper, or 花椒, comes from the seeds of species of trees in the genus Zanthoxylum. It's a spice from China's southwestern Sichuan Province. It has a unique aroma and flavor that is neither hot nor pungent, with lemony overtones. This pepper create a tingly numbness in the mouth. It is often used in spicy dishes, to create a a flavor known in Mandarin as málà 麻辣 "numb-spiciness".
When cooking sichuan peppercorns, lightly toast them and then then crush them before adding them to food. The spice is added at the last moment.