Picanha is a fresh cut of beef that's especially popular and highly prized in Brazil. In the US, it's called sirloin cap, and in the UK, it's known as the rump cap. Picanha is situated on the back side of the animal, above the butt, where it sits on a fat cap.
It's mostly used for churrasco – the meat is first grilled, then sliced off of a skewer. This cut holds very little fat in the meat, so it must be cooked perfectly in order not to make it tough. In Brazil, every churrasco has picanha, and all of the best churrascarias feature picanha on their menus.