What is a dormant black hole?

A black hole "feeds" on its environment, usually. So matter falls into the black hole. Due to angular momentum conservation the matter will start to spiral. At some point it will hit the "centrifugal barrier" and keeps orbiting the black hole. When more matter is accumulated, the black hole will create a massive ring of matter.

Tidal forces will crush the matter, friction will heat the matter. The heat will become so large that the thermal radiation pushes matter away again. Other parts of the matter lose energy and fall beyond the event horizon.

When a black hole has consumed all the matter in its vicinity, it can no longer generate radiation from its belt. At this point you could call it "dormant" since nothing falls into it.

A dormant black hole, as the name suggests is a sleeping black hole, i.e. one that is not actively swallowing matter around it. Black holes can't turn on and off, so a dormant black hole would not be in contact with any large sources of matter.

In contrast, black holes which are not dormant are actively pulling surrounding matter closer to its horizon, thereby creating a glowing hot accretion disk around it.