Play a string, or click its name (E2 A2 D3 G3 B3 E4) in the lower left. The line shows the period of the note you play; the tick under the display is the target. If your line is right of the tick, the string is low: tighten it until the line turns green. Auto Select picks the string nearest the note you play; turn it off if your guitar is far out of tune.