Even though you asked for this in Ab, I am giving it to you in C, because it a pain to play in Ab on the guitar, and it is a fairly standard thing in C, moving down in the bass to G on the guitar(people used to thing of it as the "Mr. Bojangles" progression, but it really comes from the Pachelbel Canon, a classical piece, written about three hundred years ago)

The song simply repeats this chord progression, four slow, stately beats to the measure(two counts on each chord)both verse and chorus(the slash marks divide the measures):

||:C-Cmaj7/C6-C/F-Fmaj7|F6-G7:||

Basically, you move downwards scale-wise from the bass C note,C-B-A-G, then change to the F chord, and do the same, F-E-D-D but instead of moving to the C note on the last two counts, you stay on the D note and play G7 chord--