A m7(b13) chord is a m7 chord with an additional minor 6th. Because the minor 6th is extending the 7th chord rather than replacing the 7th, it is written as b13 in the chord symbol.
The Cm7(b13) has the notes C (1), Eb (m3), G (5), Bb (7) and Ab (b13). The 5th can be omitted. Attention: if you play the 5th, take care about the voicing (order of intervals on the fretboard): the 5th should be played on a higher string than the b13, otherwise you'll end up with a flat 9 between 5th and b13, which sounds very dissonant in that context.
The m7(b13) is often interpreted as slash chord Abadd9/C or less frequently Abmaj9/C (explicitly including the 5th of C). It's so to speak the more jazzy variant of a simple E/C chord
You can here a very interesting application of this chord in the intro and interludes of Deacon Blues by Steely Dan, where it works as link between maj7 chords:
Cmaj7 Bm7(b13) Bbmaj7 Am7(b13) Dmaj7 Dbm7(b13) Cmaj7…
Using the slash chord writing (2nd variant as maj9):
Cmaj7 Gmaj9/B Bbmaj7 Fmaj9/A Dmaj7 Amaj9/Db Cmaj7…
...actually just maj7 chords connected in an elegant manner!