Watch the Polyrhythms Introduction online guitar lesson by Ben Lacy from Two-Hand Groove Guitar
Polyrhythms are just different rhythms played by each hand. Like maybe the right hand playing a steady 4 while the left plays 3, or 5, or even 7. They can get as wacky as you like.
If you keep the right hand in a steady 4 you can do all kinds of wackiness with the left, including speeding up and slowing down. One of my favorite moves is to have the left hand sound sort of drunk, unrelated to the steady 4 of the right.
The possibilities are unlimited so try some of these with me on the play-alongs.