Hydraulic pressure is transmitted through liquid. Since liquid is effectively incompressible, pressure applied to a liquid is transmitted without loss throughout the liquid. In a braking system, this allows a force applied to the brake pedal to act upon the brakes at the wheels.
Hydraulic pressure can transmit increased force. Since pressure is force per unit area, the same pressure applied over different areas can produce different forces - larger and smaller.
This graphic illustrates a system that has cylinders of different sizes. When the brake pedal is pressed, the force against the piston in the master cylinder applies pressure to the fluid. This same pressure is transmitted throughout the fluid, but it has a different effect on each piston in the other cylinders: