In improv, object work refers to miming physically non-existent invisible items within in the invisible environment in a scene.
Most improv is performed without any physical props.
Objects are important as they help create action, context, and move scenes beyond using dialogue along.
"There is nothing more boring to watch than people standing still, acting like talking statues. And doing object work is a great way of freeing you up and getting you to move around. Last weekend, I taught an improv workshop at the Out of Bounds Comedy Festival in Austin, and in the class, two girls did a scene where they were seducing a guy in their apartment. They both went to make the guy a martini in a shaker. And they shook those shakers so damn sexy that they got an enormous laugh from that action. These two improvisers were showing the audience how they were feeling through the activity, rather than telling us how they were feeling, and it was a joy to watch." 
- – Jimmy Carrane