An electronic device involves lots of different chips, components (resistors / capacitors), connectors, and so on -- maybe dozens or even hundreds of parts. The parts have pins or wires sticking out of them that need to be electrically connected. (Some chips have a couple dozen connection pins.)

A circuit board is a sturdy plastic / fiberglass surface on one side, copper on the other side [1]. A "new" circuit board has a solid copper surface on the back, coated with a light-sensitive protective chemical.

A solid surface is no good though. As a circuit designer you don't want everything connected to everything else, you want to make connections between specific points only.

So to get the connections you want, you make a shadow over the parts of the copper you want to keep, then turn on a bright light, then wash the copper with acid. The acid dissolves the copper, but only where the light destroyed the protective chemical.

Now you just have to:

• Put your components in the right places on the fiberglass side
• Drill holes for the wires / pins
• Glue ("solder") the wires / pins to the copper with a tiny bit of molten metal

After soldering, the parts are mechanically locked in place, electrically connected where they should be and insulated everywhere else.

There are many circuit board customization companies that offer these services for very cheap prices. You send your circuit design files to the company, and they use specialized robots to wash the copper, drill the holes, and also print directly on the board whatever graphics / labels you want ("silkscreening"). They ship you the finished circuit boards by mail / UPS / Fedex like any other online delivery. For an additional fee, another specialized robot can place the components and solder them for you ("pick and place") [2].

You can buy one or a few circuit boards with a unique circuit design for a prototype or a hobbyist one-off. Or you can order hundreds or thousands of identical boards at a time, if you're making a mass produced commercial product.

[1] Circuit boards can also have multiple layers.

[2] A lot of modern electronics use components ("SMT") that have very small, closely spaced connection points. These components are really designed to be soldered by robots; it's difficult for a human to solder them by hand.