Slash-Chord. On the left side of the slash you have a chord, on the right side you see the lowest tone (bass).
This chord exactly corresponds to a sus9 chord [9sus, 7sus4(9)] without the 5th being played (normally the 5th isn't played with that chord on guitar anyway). Thus C/D equals a Dsus9 chord [D9sus, D7sus4(9)].
Especially in pop music that type of chord is often used to replace a dominant 7th chord. So G7 is often replaced by F/G.