A ground control point (GCP) is a location of the surface of the Earth (e.g. a road intersection) that can be identified on the imagery and located accurately on the map (i.e. the reference dataset). Two distinct sets of coordinates are associated with the GCP: image coordinates in i rows and j columns, and map coordinates (e.g. x, y measured in degrees of latitude and longitude or as specified by the spatial reference system).
Spatial referencing (referred to as geo-referencing as well) is the process of aligning available EO or GIS data to a coordinate system so that further spatial analysis and image analysis tasks can be applied using these data as input.
To be able to perform spatial referencing, users have to generate the so called Ground Control Points (GCPs) with known coordinates. In case of images, the easiest features that could be used as GCPs are the intersections, isolated trees etc.