Short answer
HubSpot's KB: deal scores use AI on deal properties and activities to predict the probability of winning open deals. A score is a percentage. New deals usually get a score within about 36 hours. Closed deals stop updating, reopened deals get no score, and setting probability to 0% or 100% without closing still leaves a score.
1. The property
Deal Score in my demo portal's deal properties.
2. Thresholds
HubSpot doesn't set a good-score threshold by stage, and says scores shouldn't be the only factor in decisions. See stage probability.