This term dates back to before there was
television and before there were
movies but they still do it. When an actor is doing something on stage other than
crossing or actions specifically put into the
script in order to add believability and realism to their
character, they are performing stage business. It's usually simple things that we take for granted as
real people. Like playing with the phone cord while pretending to talk on the phone, or
doodling on a pad while taking notes as their character questions a suspect; provided it's a murder mystery. Or they could be
flipping a coin or playing with their hair.
There's no end of possibilities.
The Picard Maneuver is an example of stage business as performed on Star Trek: The Next Generation.