For some Open Hardware is a logical evolution of the Free Software philosophy applied to physical stuff, where both code and design blueprints co-exist. However no one can deny that humankind has evolved through a DIY and hack & share-it culture until patents prevented us to do so.
Hardware in our definition is used in its full meaning and covers not only electronic products but also stuff which do not run code such as furnitures and any "low tech" physical object you can think of.
For the scope of Hardware Freedom Day Open Hardware is a term used to describe physical objects which design is created and shared publicly without restriction, allowing people to modify, improve and redistribute their contributions. The design includes drawings, blueprints, software code that may be running in the device or used to generate some of its parts or drivers and any other information one could need to create that device itself. Generally this information is available under an electronic format on individual project websites, driven through the use of the Internet and rely on community knowledge and local DIY spaces (also often called hackspaces) to see the day of light.
SparkFun Electronics has created a very nice explanatory video for the Open Source Hardware Association which you can view here.
Goals and philosophy of the Open Hardware movement are closely aligned with the ones of the Free Software movement.
For more information you probably want to read what wikipedia has to say about the topic as well as their page on Open Design. Then if you are still interested a visit to Hackerspaces.org: