The following image shows the i-ii^o6-V-i chords in the key of A minor. Lowering the root of the ii chord by a semitone (changing the B to a Bb in this case) creates a Neapolitan sixth chord.

In traditional harmony, chords in first inversion are sometimes referred to as sixth chords. By lowering the root note, we increase harmonic tension and strengthen the subdominant function and resolution to the dominant chord. The lowered note then descends a diminished third to the third of the dominant chord. Listen to both examples:

The Neapolitan sixth chord is often followed by the tonic chord in second inversion, which then leads to the dominant chord.