Base material is the carrier of the assembly and circuits of printed circuit boards. The base material is delivered to the PCB manufacturer as "panel" and will be cut in the appropriate production panel size.
There are many differnet PCB base materials with different thicknesses and laminations as well as electrical and mechanical properties, which affect the function of the electric circuit. See also PCB materials.
Often the base material is made of fiberglass with epoxy (FR4) which is available as core (copper-clad) or prepreg.