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.

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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.