Stage business is the combined small
movements and actions that an
actor uses to help build information for the
audience. If someone is nervous, having someone sit there and say they’re nervous is not convincing. Having an actor
fidget and look around nervously, however,
shows rather than tells the audience that the character is nervous.
An example of this can be seen in scene eight of A Streetcar Named Desire. When Stanley blows up at Stella, we find that he is a crude, abusive man. Some of the stage business that Tennessee Williams scripted (throwing a plate on the floor, roughly grabbing Stella by the arm) shows the audience this, rather than having Stella just saying so outright. This builds in realism, which in turn draws the audience into the story.
Stage business tends to be the one thing missing from high school plays. Most of the ones I have seen had half of the characters just hanging out spouting lines. If I had my druthers, that would be one area I would focus on should I ever find myself behind the director's chair.