Polychords are just 'more than one chord' chords.... :D

The most common use of them in Guitar is the 'slash' chords - playing one chord with a different note in the bass. Best example I can think of right now is playing an open C Major chord with the bass on G (Third fret of sixth string instead of third fret of fifth).

That's a poly chord. Pianists use them differently - playing a different chord with each hand. We can do that too by tapping triads with both hands. Bumblefoot does this with some interesting results...

A Polychord does not need to have more than 7 notes...