Knock Code or Tap code is an easy to learn encoding of 25 letters of the alphabet into knocks/flashes. Space plays/pauses, any other letter is taken as a guess for the next letter that you haven't guessed yet in the word. If you pause then play, play will continue from the first letter you haven't guessed yet. If you've guessed all the letters, a new word will be chosen.
Letters are encoded into groups of two. The first set of taps selects between a/f/l/q/v, the second set selects between the 5 letters starting with the letter chosen in first set, so 'o' is encoded as 3 taps then 4 taps and 'a' is encoded as 1 tap then 1 tap.