If you have never come across such a device, it is simply a way of determining how to solve for any of three variables in an equation where two of the variables are multiplied to give the third value. In the first pyramid above, if you wanted to predict volume (V) and were given mass (m) and density (d), you would use your thumb to cover up the V in the pyramid (what you want to predict). Doing so leaves m over d - indicating you calculate mass divided by density.
Want to determine mass? Cover the m, and you see that you would multiply density and volume, since they are side-by-side.
Is it easy? Yes. And that's why many students (and teachers) gravitate toward using them. My issue, though, is that their usage relies on memorization (you have to get the pyramid right before you can use it), and it removes algebra from the process. Sure, a pyramid works very well for "rearranging" an equation like V = IxR, but what about SA = πrs + πr^2? And don't even get me started on the complete lack of unit analysis...
When I was actively teaching concepts like Ohm's Law (in the circle, above), I never mentioned the pyramid, and made all my students rearrange the equations using opposite operations. I've always insisted that they practice the "hard" way, to better prepare them for the day they need to solve more complex equations. I've even been known to "boo" loudly (with a smile!) if a pyramid ever showed up on a student's work.
But now I'm letting the students choose how they learn concepts like Ohm's Law, and even though the majority of the resources I provide (including the textbook) show no sign of the pyramid, lo and behold, the dang things are showing up EVERYWHERE.