A plane, in a networking context, is one of three integral components of a telecommunications architecture. These three elements -- the data plane, the control plane and the management plane – can be thought of as different areas of operations. Each plane carries a different type of traffic and is conceptually (and often in reality) an overlay network (a telecommunications network that runs independently on top of another one, although supported by its infrastructure).
The data plane (sometimes known as the user plane, forwarding plane, carrier plane or bearer plane) carries the network user traffic. The control plane carries signaling traffic. Control packets originate from or are destined for a router. The management plane, which carries administrative traffic, is considered a subset of the control plane.
In conventional networking, all three planes are implemented in the firmware of routers and switches. Software-defined networking (SDN) decouples the data and control planes, removes the control plane from network hardware and implements it in software instead, which enables programmatic access and, as a result, makes network administration much more flexible.
Moving the control plane to software allows dynamic access and administration. A network administrator can shape traffic from a centralized control console without having to touch individual switches. The administrator can change any network switch's rules when necessary -- prioritizing, de-prioritizing or even blocking specific types of packets with a very granular level of control.