When I talk about polymeters in clinics and workshops it can garner some blank stares. It makes sense. Many drummers haven't spent much time thinking about or studying polymeters.
I'd like to give a simple explanation for you t begin thinking about. If we hit the concept from a few different angles it will not only click, but it will also start you on a path of study that will lead to a whole new territory of rhythmic possibilities. So let's take a quick look at the polymeter.
A polymeter is two or more meters happening at the same time. These meters share a common subdivision, so in essence, a polymeter is different groupings of the same note values being played alongside one another. For example, 3 sixteenth notes being played over and over at the same time as 4 sixteenth notes are being played over and over. They begin together, then grow further apart in their starting note until finally (3 beats later in this case) they re-align and begin their rhythmic cycle again.
You probably hear the term polyrhythm much more than polymeter. They are related in a "mirror image", "upside down", or "inversely proportionate" sort of way.
A polyrhythm is two or more evenly spaced note values with different subdivisions resolving within the same amount of time. For example, four quarter notes being played in the same amount of time as 3 half note triplets. Because a quarter note is typically counted in duple subdivisions it is different than a triple subdivision thus creating a polyrhythm rather than a polymeter (which has the same subdivision).
In the video above, you see and hear four cymbal notes in the same amount of time that you hear 3 snare drum notes. You can also see that the pulse is counting one way for four measures, and then another way for four measures. Practice shifting your rhythmic perception as you listen to the ride and snare and follow along with the two different counting systems.
When you are counting to four, you are in 4/4 playing quarter notes in the right hand with a triplet subdivision of half-note triplets in the left hand. When you are counting to three, you are in 3/4 with the left hand playing quarter notes and the right hand playing dotted eighth notes (or groupings of three 16th notes).
Once this starts to make sense from watching the video, try to tap out the pattern and count along aloud (it's very important to count aloud and not just to yourself).
This is a great workout. Don't only limit the coordination to your hands. Try it with all two limb combinations: RH/LH, RH/LF, RH/RF, LH/RF, LH/LF, RF/LF (and then switch the role of each limb...).