In simple terms, "hybrid" means the use of two or more major systems, like a hybrid car that uses a gasoline engine and an electric motor for power. In the timber-frame world, it refers to the structural systems. You have to remember that aside from being an aesthetic element, a timber frame is structural — it's holding up the building and supporting the enclosure. If the design is a hybrid, the structural work is being shared between two or more structural systems. For instance, a hybrid timber frame and structural insulated panel (SIP) building will have some structural loads borne by the timber frame and some carried by the structural panels. This hybrid system can mix the timber frame with any other structural system, i.e. insulated concrete forms (ICFs), conventional stick framing, steel framing, log, or rammed earth and straw bale. A hybrid timber frame just means that the heavy lifting is done by a mix of timber framing and some other system.