It's not exactly a new concept. Walking nachos, walking tacos, fritos pies, and the like have been around since, at least, when I was a kid but it's more of a do-it-yourself kind of thing. Taco Bell's version is basically nachos in a bag. It includes many of the ingredients you'd expect but also features new tortilla chips made specifically for the menu item.
Other than the new chips, Taco Bell's Walking Tacos include seasoned beef, nacho cheese, shredded 3-cheese blend, sour cream, pico de gallo, and guacamole. Notably missing are beans. Basically, if you like Taco Bell's Nachos Bell Grande, this is a more portable, on-the-go version.
Taco Bell's Walking Nachos are set to begin testing starting mid-August over in Charlotte, NC.