Just bought my first street bike. Need some input on a Petcock?

The guy who sold me the bike told me that the valve that controls fuel flow was worked on and the positoin of it is that the gas is always on. I'm guessing its a Petcock valve. an explaination please of what he is talking about

In a traditional motorcycle configuration the gas tank is in front of you in-between your thighs and the handlebars, on top of the engine and above the carburators. Somewhere off the bottom of the gas tank was where at least one 3-position fuel valve (or petcock) is located.

There are 2 different types:
1) The original type, a manual 3-way valve, and
2) The newer type that incorporates a vacuum diaphragm.

The old ones required that you remember to turn them "off" when you get turn the key off and get off to leave the motorcycle parked. The old danger was that your gravity feed fuel system could keep flowing and over flow the carbs and seep liquid fuel onto the top of the pistons, and then seep past the piston rings and mix with your crankcase oil. All of these things can destroy your engine either suddenly with a "bang" or slowly over a few hundred miles.

The newer gas tank fuel valves that required vacuum from a spinning engine to function were safer, and not susceptible to human error and forgetful memories.

to put it simply, it is a petcock, chances are( unless you have an actual fuel guage) it has 3 positions, on-off-reserve, it sounds like someone worked on the petcock and now all it has is "ON". If you dont have a fuel guage might want to replace the petcock so you dont run out of fuel one of these days