A BETTER GUITAR SETUP
What this check measures
The fretted note at the 12th fret should sit one octave above the open string. The tool first measures a correctly tuned open string, then compares the fretted pitch against twice that measured frequency.
Keep the comparison consistent
Remove your capo. Mute neighbouring strings and use normal fretting pressure. Measure the same string twice, then repeat the whole comparison. Pressing too hard, old strings or an unstable note can change the result.
What the result means
A positive difference means the fretted note is sharp relative to the open note. A negative difference means it is flat. A persistent difference may point to an instrument setup issue. This check cannot identify the cause or decide which adjustment your guitar needs.
Further reading: Sam Ash: understanding guitar intonation