On some tabs, I sometimes see a "NC" text on top of the tab.
What does it mean ?
How should it be played or interpreted ?
NC (or N.C.) is short for "No Chord".
It means that you should only play the indicated notes or melody, and not try to infer or add a chordal accompaniment. This is as opposed to the chord symbols that you probably find everywhere else than where the N.C. notation is.
See for example
(Although their example doesn't use tabs.)