Ukuleles go out of tune all the time: new strings stretch for days, and a change of room is enough to pull them off. Learning to hear it means you stop strumming along to a uke that sounds slightly off without knowing why.
In Tune? plays the four strings (G C E A) one after another. Is it in tune, or out? Half the rounds are spot on; in the rest, one string or more has slipped. Press Strum to hear the chord. Strings that are out make it wobble and sound sour.
Start on Easy (a quarter-tone out), then try Hard (about a fifth of a semitone).