It’s an exercise program that you can buy at Walmart or other big stores like that.
It comes in DVD’s and pushes you to exercise in a high intense fashion, as opposed to lower intensity exercises, but longer workouts.
In principle, those who stick it out will become stronger and more fit using it.
P90x is not available at the Walmarts here. (Perhaps it was regionally released???), but there is a $30 knockoff being sold at Target (The title of the knockoff is “Supreme 90 Day Workout”, IIRC.)