Boost is basically "rubber-band racing." Say there are 5 drivers, the 1st driver does not get any boost at all as he doesn't need it, 2nd gets just a little bit, only enough to catch up to 1st and the far back you are in the pack the more effective boost is for you. If you last you will get the full effect of the boost.
What boost does it give your car extra grip in your tires and lowers wind resistance so your basically in a 100% draft all the way to first place. You will notice you car can take corners faster and go faster on straights as well as faster acceleration. Your car will basically be a car on rails with a rocket on the back of it.
Difference between Low Boost and High Boost is basically how quickly it comes into play and how noticeable it is. Low boost you will hardly notice it because it doesn't give you that much more of an advantage but it gives you some. High Boost is basically a god send, it will be like your drive a super car. If you're already driving a super car, then it'll feel like your driving an X2010 (not really but you get what I'm trying to say).
To get an idea of how effective boost is, do an online race on Daytona without boost and then a race with it. Make it a 10 lap race and spin out on purpose. You will see that with boost on you will be able to catch up to the leader in just a few laps, but in the race without boost, you will just be a lone wolf on the race track until you can catch the draft of another car.
Hopefully you can understand what I said.