The term polyrhythm denotes the rhythmic dissonance created by the simultaneous use of two or more conflicting rhythms in different time signatures. This can be systemic, the basis of an entire track – cross-rhythm – or just a brief momentary disruption, e.g. the hi-hat or snare triplets rolls on a trap beat.
Try it out yourself: Program a drum loop, set the kick and snare in a straight 4/4 rhythm, but put hi-hats in a 6/8. Add a bassline pattern in 4/4 and put some chords in 5/4, voilà! (You may have to set up your sequencer or piano roll grid to 48th notes.)
With polyrhythms, this loop just got more interesting, less predictable and got some character.