Before you can tune a guitar by ear you have to notice it's out, and with six strings, one slightly flat string is easy to miss until a chord sounds sour.
In Tune? plays the six strings (E A D G B E) one after another. Is it in tune, or out? Half the rounds are spot on; in the rest, anything from one string to all six has drifted. Press Strum to hear them together. The chord wobbles when something's off.
Start on Easy, where a bad string is a quarter-tone out, then move to Hard, about a fifth of a semitone, the small drift a guitar picks up between practice sessions.