The Break Fall Roll is very important when learning parkour, because if you fail while practicing another move and end up falling, your muscles should be trained to automatically handle a fall accordingly.
The idea of this roll is that instead of letting the impact hit one part of your body and get a potent injury is that you spread the impact across a larger surface area of your body to mitigate the impact.
It's not a somersault like you do in gymnastics, because if you tried that from a large height, you could get a bad head injury.
Instead, what most people do is they choose which diagonal shoulder-to-lower back direction to roll in. The surface area that will mitigate the impact is diagonal from your shoulder to your lower left back. What most people like to do is use their hands to help stabilize them as they go into the roll. Keep your head tucked in so you don't hit it.
It's good to practice this on a soft surface like grass, or if practicing indoors, it's wise to use a large carpeted floor with enough room to roll around in.

This is my favorite part about cosplaying as Ezio from Assassin's Creed on this forum.