Complex sentence: After two-layer PCBs, the next step up is usually four-layer; of the four layers, often two are dedicated as power supply and ground planes, respectively, and the other two are used for signal wiring between components.
Split into simpler sentences: After two-layer PCBs, the next step up is usually four-layer. Often two layers are dedicated as power supply and ground planes, and the other two are used for signal wiring between components.