Doides serve as one-way valves for current. They are made of two layers of different materials.
Electrons can easily pass in one direction, but not so easily in the other.
If you connect 4 diodes, you can turn AC into DC. This setup of diodes is called a bridge rectifier.
Below is an image showing an AC source supplying current to a bridge rectifier. The two wires coming out at the right are now DC.