A Measured roll is a way of keeping track of the beat that you are playing when you are rolling and depending on the tempo you can roll in either a duple or a triple subdivision.
Watch this Video Lesson on How to Play Measured Rolls
This is how to think of playing duple subdivision:
And this is triple subdivision:
When you practice these two exercises make sure that you keep the base rhythm steady. Don’t change what your hands are doing; just let the sticks bounce when you see the double slash on the note stem and don’t let the sticks bounce when the note stems are plain.