Polychords (or slash chords as they are sometimes known) may sound fancy, but are really quite simple. They are basically just major or minor chords with a different bass note added to the chord.
They are written, for example, as C/B, where C is the chord you play (C Major) and B is the bass note added to the chord, usually replacing the root note.
Here is how this chord could be fingered:
Polychords can also be viewed as the 4th inversion of a 7th chord (see Seventh Chords).