As the name implies, it is a bologna based taco. However the key difference is instead of using a taco shell the slice of bologna is being used as the taco shell. The toppings such as mustard are then directly applied to the bologna.
It is one type of a poorman's sandwich.
Dude: Hey man, I am starving.
Guy: I am outta bread, so go make yourself a bologna taco.
Dude: I better not get listeria.