Tuning a guitar by ear comes down to two questions: which string is off, and is it too high or too low? This game trains both.
Out of Tune plays the six strings with one of them out. Tap the string you think is wrong, then say whether it was sharp or flat. Compare again plays it beside how it should sound, so you can hear the difference.
Easy puts the string a quarter-tone out; Hard brings it to about a fifth of a semitone. It's the same listening you use for the fifth-fret tuning method, or for checking that a tuner got it right.