Slash Chord. On the left side of the slash you have a chord, on the right side you see the lowest tone (bass).
Here we have a C major chord with B in the bass. B is the major 7th (maj7) of C. You might therfore write this chord as Cmaj7/B. Since the bass is explicitly written on the right side of the slash, it doesn't have to be mentioned in the chord symbol on the left. It's enough to write C/B.
This chord is often used in ballads, where a descending bassline but little harmonical movement is desired, for example in the chord progression CC/BAm7 C/G.
You may have recognized that the Am7 chord can also be seen as C/A, so this chord progression is actually just a C major chord over a descending bass line.
A good example is the song By Your Side from Sade. This song is originally in the key of B natural:
| B B/A# | G#m7 B/F# | E | F# | …
which in the key of C equals:
| C C/B | Am7 C/G | F | G | …