A tuner tells you what's wrong, but in a session you need to hear it yourself: a string that's drifted between sets, or a banjo that sounds sour against the fiddle.
In Tune? plays the four strings, G D A E, one after another. Is the banjo in tune, or is something off? Half the rounds are spot on; in the rest, anything from one string to all four has slipped. Press Strum to hear them together. A string that's out makes the chord wobble.
Start on Easy, where a bad string is a quarter-tone out, then move to Hard, about a fifth of a semitone, the kind of small drift you'd want to catch before the first tune.