Liquids are used in hydraulic systems to transmit forces from one part of a machine to another.
Hydraulic Press
An hydraulic press acts as a force multiplier. An effort force on piston A1 puts pressure on the liquid. The pressure is the same at all places in the liquid, which means that the same force acts on piston A2. Piston A2 is twice the size of piton A1, so the load force exerted by piston A2 is twice as big as the effort force, because piston A2 has twice the area of piston A1, and therefore only moves half the distance moved by piston A1.
Braking Systems
Car braking systems use hydraulics to multiply the force that the driver applies to the brake pedal. A piston attached to the brake pedal applies a force to the hydraulic fluid in the master cylinder. This increases the pressure in the fluid, which is transmitted to the wheels. Pistons near the wheels push pads against discs attached to the wheels. The friction between the discs and the wheels slows the car down.