The Green Burrito is a secret item available from Taco Bell. It is a bean burrito with onions, cheese, guacamole. and Verde sauce.
The burrito is called a "secret item" because it is not on the menu, and must be asked for at the counter. The "Green Sauce", for which the burrito is eponymous and notorious for, is Taco Bell's "Verde sauce", which is readily available in packets at the after-order stations inside your local Taco Bell restaurant.