A Blueprint is a specific definition that is used to create instances (objects) from.
For example, lets say you are designing a town which has a force of Town Guards who all need to have the same number of hit points, are equipped with the same items, and basically need to respond in the same manner when spoken to, when attacked, etc. So you first would create a Town Guard blueprint with all those characteristics and properties in place. From that single blueprint, using the edit copy feature of the toolset, you could then create Joe the Town Guard, a rather stupid fellow who patrols the waterfront. You could also create Sam the Guard Captain from that same blueprint, perhaps giving him better weapons and armor. Using blueprints saves the builder from having to repeat the same processes over and over while designing their module.