Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used to transfer force into pressure, and to amplify braking force. It works because liquids are not appreciably compressible.
The brake fluid your vehicle requires is printed on top of the brake fluid reservoir cap. Common brake fluids are Dot 3, Dot 4, Dot 5 which is a silicone based fluid. Dot 5 is different from the series of DOT 2, 3, 4, 5.1.