sleeper a good thing or a bad thing?

I just started boxing about a week ago and I train at a gym in fort worth. The guys at my gym have trained 6 pros, including aylala (spelling?). Im 15 years old, 170 and 6'1 (sounds skinny i guess, but im buffer than about 95 percent of kids my age and my arms are 15 inches) my main trainer trianed the 1988 gold olympian boxing team, so i guess he knows what hes talking about. anyways, im really new to boxing and ive got my punches down and whatnot, and they seem to be excited about me because ive got "one hell of a sleeper" and always pull people around the ring and show them my right straight but i dont know what a sleeper is and im too embaressed to ask haha. can someone fill me in?