Throughplating electrically connects (opposite) layers of the circuit board (see via). For this a hole is drilled through the board. Because the base material (e.g. FR4) is electrically isolated, a conductive layer must be applied electrolessly prior to plating.
The two most common methods are the deposition of copper onto palladium nuclei or flooding with graphite solution - after drying a carbon layer works as a conductor. In any case, the copper layer is then brought galvanically to a thickness of about 20µm - 25µm.