A blueprint is basically a map of a building. It is usually a two dimensional representation of where the rooms are, how big they are, with the locations of key features like windows or doors. Some blueprints show where other infrastructural features are located, such as electrical wiring, plumbing or duct work.
As the image shows, blueprints used to be printed in dark blue ink on lighter blue paper -- hence the name.
- Wikipedia's article on blueprint