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
http://dictionary.onmusic.org/terms/2344-no_chord
(Although their example doesn't use tabs.)