A minor 13th – m7(13) or abbreviated m13 – is a m7 chord with an additional major 6th. Because the 6th is extending the 7th chord rather than replacing the 7th, it is written as 13 in the chord symbol.
A Cm13 has the notes C (1), Eb (m3), G (5), Bb (7) and A (13). In practise you'll often omit the 5th, because you take it and go up a whole-step (2 frets) to play the 13.
Minor 13th chords are almost too rare to be listed here. However, for some musical styles they might be an option to add an interesting quality of sound.