The alphabet backwards
Write the alphabet forwards and backwards, one above the other, and swap each letter for the one opposite: A and Z, B and Y, C and X, through to M and N in the middle. WIZARD becomes DRAZIW. The swap is its own undo and there is no key, so a message in Atbash is read with the same step that wrote it.
Where it comes from
Atbash began with the Hebrew alphabet, and its name is made from the first and last letter pairs, aleph and tav, bet and shin; it turns up in the Book of Jeremiah. Today it is a puzzle favourite in escape rooms and treasure hunts, and was one of the codes hidden in the end credits of the cartoon Gravity Falls.
Questions
Can Atbash be combined with other codes?
Yes, puzzle makers often stack codes. Atbash and then A1Z26, for example, turns A into 26 and Z into 1. Do one step, press the swap button, then pick the next code in the Secret Code Translator.
What happens to numbers and accents?
They are left as they are. Only the 26 letters A to Z are swapped, capitals or small letters.