Amateur Question: Meaning of "Heavy"

Hello,

I'm not a pilot by any means, but had a question that I thought I
could get answered here. When I fly on UAL I sometimes listen to the
cockpit radio on channel 9 from a general interest. Usually the pilot
will identify the flight by the airline and flight number (e.g. United
282), but sometimes (only on 747s in my experience) they add the word
"Heavy" (e.g. United 192 Heavy). Obviously a 747 is pretty heavy, but
what is the meaning and logic behind this? Does it apply to other
types of aircraft? Just wondering and thought someone here would
know.

Thanks