PCB stands for printed circuit board and it’s the foundation of many electronics. It provides electrical connections between components on the circuit, including resistors and capacitors.

These connections can be made with a single layer of copper, but when you build more complex circuits, you need to add more layers of copper to accommodate additional connections. When you build a double-layer PCB, you’re stacking two layers of copper together in order to create more complex circuitry.

Hence, a double-layer PCB is one that has two copper layers. This is a common technique used in making circuit boards, and it can be used to help reduce the number of connections through the board, as well as increase its flexibility.