In music theory, a Neapolitan chord (or simply a “Neapolitan”) is a major chord built on the lowered second (supertonic) scale degree.
A Neapolitan chord most commonly occurs in first inversion so that it is notated either as ♭II6 or N6 and normally referred to as a Neapolitan sixth chord. In C, a Neapolitan sixth chord in first inversion contains an interval of a sixth between F and D♭. In Schenkerian analysis, it is known as Phrygian II. In doing roman numeral analysis, it is sometimes indicated by an “N” rather than a “♭II”. Putting aside music theory of Neapolitan chords, explained very nicely in this wikipedia article, all you have to remember is that Naepolitan chords are simply major triads constructed on a special note.
To understand how to build a Naepolitan chord, there are three steps involved: