Fuses

Fuses, in a basic sense, have a thin piece of wire inside them, which is rated to handle a certain amount of current flowing through them. In this case, this one is rated to handle three amps or 19 quintillion 272 quadrillion electrons per second. If too much current flows in the circuit, then the fuse will burn out and this will open or break the circuit to protect the expensive electrical components, the fuse acts as a weak point and is very cheap to replace.

So you can find these mounted on circuit boards. And in the UK, plugs will often have a fuse built in for added safety.