What is it called when you ride around on your front wheel?

Rolling along balancing on the front wheel is called a "nose manual". If it's braking to a stop while balancing on the wheel it's a "stoppie". If it's braking to a stop suddenly with the rear wheel lifting and no front wheel rolling it's called an "endo".

It has many different names. Its usually called a stoppie or nose manual. Lots of people also call it a "fakie", but that term is more common in skateboarding. Some people refer to it as an "endo" but that's usually used to refer to a crash where you go over the handlebars. Other names such as nose rides, front wheelies, tip ups, and tip rides are used, but its pretty rare that someone calls them that.