Fm chords is to use the root six major bar chords. By substituting these shapes it is possible to use the sixth and fourth strings as bass notes thus allowing the right hand fingers to play the third string. Note that the note that is normally played on the 3rd fret fifth string is not needed as the right hand thumb mostly picks the bass notes on the sixth and fourth strings.
This fingering also frees the fourth finger of the left hand to play other notes based around the chord. The root six seventh bar chord is also frequently used. These chord shapes apply to other root six bar chords positioned on higher frets such as F#, F#m, F#7, G, Gm, G7 etc.