A hot lead is a buyer who is close to acting: a short timeline, financing in place or nearly so, and no agent of their own. At an open house, the hot leads are the small share of visitors worth calling the same day, before they tour three more homes.
The point of qualifying at sign-in is to find these people while the context is fresh. Warm and cold leads still get followed up, but the hot ones set the order. Miss the same-day call and a hot lead cools fast.