Nashville tuning (or “high stringing”) a guitar involves replacing the heavier strings with a lighter guage, and tuning them an octave higher. Chord shapes stay the same, but the guitar produces some notes an octave higher (somewhat like a 12-string.)
Without the lower strings, the tone lacks body, and as a standalone guitar track a Nashville-tuned guitar would sound thin. But a guitar with this tuning can be used to double a normally-tuned guitar track, and brighten the sound.
Here’s an example:
Now here’s the same riff played with two guitars, one tuned normally and one high strung:
(The riff is from the song Funny Feeling.)