Slash-Chord. On the left side of the slash you have a chord, on the right side the lowest tone (bass). The Cm/F (say „C minor over F”) is a C minor chord with a low F played in the bass.
The Cm consists of the tones C (1), Eb (3) and G (5). In a slash chord the bass can sometimes be part of the chord, but doesn't necessarily have to. In this case the bass note F is not a part of the Cm chord written on the left.
The Cm/F chord can also be considered as F7sus2 chord. Or as F7(9) without the 3rd being played. Therefore you can play this chord instead of F7 (F9, F13) or F7sus (sus2, sus4, sus9, etc.). Practically it will most likely be used instead of Fsus9 (and you can normally as well apply Fsus9 where Cm/F is written). Fsus9 can be interpreted as Cm7/F (considering the 5th C of Fsus9 being played). Usually you'll more likely interpret/write Fsus9 as Eb/F.
Of course you may also stumble across this chord, where several chords (mostly major and minor triads) are played over a static „pedal bass”.