When your ukulele sounds wrong, the quickest fix is to work out which string is off and whether it needs to go up or down. That's an ear skill, and it's what this game trains.
Out of Tune plays the four strings (G C E A, with the high G) with one of them out. Tap the string you think is wrong, then decide whether it was sharp (too high) or flat (too low). Compare again plays it beside how it should sound.
Easy puts the string a quarter-tone out; Hard brings it down to about a fifth of a semitone. Once you can hear that, tuning by ear gets quick, and so does spotting when a clip-on tuner has misread a string.