Your vehicle’s brake pedal is attached to the brake master cylinder via a rod. The purpose of a brake master cylinder is to apply hydraulic pressure using brake fluid to each wheel brake of your vehicle.
The plastic reservoir on top is where you add brake fluid. A common mistake is “topping off” the brake fluid when it starts to drop. Don’t do that. Only add fluid if your vehicle has a leak and the reservoir has run dry. Then get it looked at right away.
The reason you don’t want to add fluid is because there is usually a sensor in the the brake fluid reservoir that turns a light on when the fluid gets low. This means there is either a leak or the brake pads are getting thin and it’s time for replacement. The only sure thing is at this time get the vehicle inspected.