We learned before about electrical circuits.
If you take apart something electronic like an alarm clock, computer or a radio,
you will see a colored board with a bunch of metal and plastic things sticking out of it.
This called a circuit board.
(from: wikipedia - printed circuit board)
The lines that you see on the circuit board are made of metal that are like wires in a circuit,
laid out on the board as small as possible to save space and make electronics really tiny!