It is an Ab seventh chord, spelled that way because of the key signature.
In C minor, the dominant is G, so it occurs in the music as often as C (this is in basic stuff, not harmonic ventures and the like)
Composers want to keep that note natural (common practice, I guess) so they write a chord with Gb in it as an F#
Alternative: the Ab7 chord could lead to G7, in which case the same principles apply: namely, saving accidentals from being used too much.
Hope that helps clear it up.
Phil