Polychords are large chords that can contain six or more tones. On the piano you play a chord with the left hand and another chord (called Upper Structure) with the right hand.
You can play the upper structure over any chord quality, (major, minor, etc) but; in jazz practice the upper structures are commonly played over a Dominant chord.

An example would be a G13(#11) Polychord:
G7=[G,B,D,F] and A=[A,C#,E]

G13(#11)=[G,B,D.F,A,C#,E]

Usually other instrument plays the bass tones and the guitar plays the upper structures, however; you do not need the assistance of other instrument to play Polychords on the guitar. You can leave the 5th out of the Dominant chord. Here are a few G13(#11) voicings:

e----------0--------------0--------------12------------------------------
B----------0--------------2--------------12------------------------------
G----------0--------------4--------------10------------------------------
D----------3--------------3--------------11------------------------------
A----------4--------------0--------------10------------------------------
E----------3--------------3----------------------------------------------


Hope it makes sense.