ETAOIN SHRDLU is a made-up phrase, or name, coined as an in-joke by the operators of Linotype machines. It's composed of the twelve most often used letters in the English language: e, t, a, o, i, n, s, h, r, d, l, and u. Because of their frequent use in typesetting, these letters appear on the first two rows of keys on Linotype machines.
The 1978 short film, Farewell, Etaoin Shrdlu, documents the final day of Linotype pressing at the New York Times before the newspaper switched to computer typesetting. If you'd like to know more about Linotype machines and their role in the history of computers, you can watch Farewell, Etaoin Shrdlu on Vimeo.