Medicine balls are weighted balls used for a number of exercises. In P90X2 they are mainly used as a tool to make push-ups harder, but med balls can be used do develop the core and in a a number of explosive-type training moves. I have yet to find Medicine Balls a really like or would recommend, so see if you can’t see a few samples at your local sporting goods store before purchasing some.
Medicine balls do come in different materials (some are leather and others rubber) and weights. So, make sure you get the right finish and weight for you.
For P90X2 I’d recommend buying at least 3 balls (2 of the same weight and 1 of a heavier weight). I personally purchased two 8 lbs balls and one 10 lbs ball.