A dead key is a key on a computer keyboard that produces no output when it is pressed, but modifies the output of the next key pressed after it.
Dead keys are commonly used to generate letters with accents (diacritics), because that way one does not need one key for each possible combination of letter and accent, but only one key for each accent (the dead key) plus the usual letter keys.
For example, if a keyboard has a dead key "´", the French character e accent aigu (é) can be generated by pressing first "´", then "e".
This functionality resembles the way diacritics are put on letters in a typewriter. In such machines, when you type a diacritical mark, such as "´", or "^", the carriage won't move unless you press space or type a letter below the mark, just like a dead key. Clearly, this has no restrictions, so you could accentuate a "t" for example.