I believe a polyrhythm is when you play an uneven beat on top of an even count. The most common example of a polyrhythm that I've seen is playing triplets over a duple beat or a quadruple beat.
For example, when you have a typical 4/4, you could play an even count over the beat, like 2 beats per bar or 4 beats per bar. If you want to play in a polyrhythm, you can play a count that doesn't divide over it evenly, like triplets.
One great example is Mitch Mitchell's opening drum fill in Little Wing. He comes in at about the 30 second mark, and he starts it off with four triplets over a 4 count. Over each beat he hits three notes instead of 2 or 4 as most drummers would tend to play.
Playing in polyrhythms is a great way to keep your playing from sounding too stiff.