In electrical engineering, a ground plane is an electrically conductive surface, usually connected to electrical ground. a ground plane is a large area of copper foil on the board which is connected to the power supply ground terminal and serves as a return path for current from different components on the board.1)
A ground plane is often made as large as possible, covering most of the area of the PCB which is not occupied by circuit traces. In multilayer PCBs, it is often a separate layer covering the entire board. This serves to make circuit layout easier, allowing the designer to ground any component without having to run additional traces; component leads needing grounding are routed directly through a hole in the board to the ground plane on another layer. The large area of copper also conducts the large return currents from many components without significant voltage drops, ensuring that the ground connection of all the components are at the same reference potential.
Going to add a tutorial for adding Ground Planes in eagle. Also will link to Eagle Polygons.