The term is used a lot in sports medicine in Australia. Physio therapy, for instance. When re-habbing, or building some muscle for a purpose (like correcting an imbalance, or to protect something), co-contraction through a movement can build and train the muscle or muscle group.
Basically, you are tensing/activating a particular muscle, while you do something with that part of the body. It might be a muscle that is being over-ridden by another muscle, so you have to actively turn that muscle on, to overcome problems caused by this. Sometimes it takes a while to learn how to isolate and control that particular muscle.