The brake fluid reservoir in your car is the place that stores the brake fluid. So that when you push the brake pedal, there is fluid available to do work. This reservoir can either sit on top of the brake master cylinder, be an integral part of it, or be attached by hoses and mounted to the inner body panels. Checking the fluid level on most newer cars is just a matter of looking at the reservoir and verifying that the level is within the marked acceptable range. On older cars with integral metal master cylinders, you will have to unlatch the locking bail to remove the cover and look in.