"Software-defined networking (SDN) is an approach to computer networking which allows network administrators to manage network services through abstraction of lower level functionality. This is done by decoupling the system that makes decisions about where traffic is sent (the control plane) from the underlying systems that forward traffic to the selected destination (the data plane). The inventors and vendors of these systems claim that this simplifies networking.
SDN requires some method for the control plane to communicate with the data plane. One such mechanism, OpenFlow, is often misunderstood to be equivalent to SDN, but other mechanisms could also fit into the concept."
For a more basic understanding (which still requires some background knowledge of the three conceptual planes of operation for a switch or router - Forwarding Plane, Control Plane and Management Plane) you might check out this article. http://packetlife.net/blog/2013/may/2/what-hell-sdn/
In simple terms, SDN means decoupling the control plane from the forwarding plane and offloading its functions to a centralized, software based controller.