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A liquid is not compressible .
When you press with a plunger diameter 10mm. in a closed and filled up wilt a liquid (eg hydtaulic oil) and move the small plunger 100mm. You have a (pipe) connection to a bigger cylinder with plunger diameter 100mm. Than the this bigger plunger will move only 1mm. however the mechanical force will be 100x as strong (friction not taken into account)
The oil pressure (force) is in ratio to the surface of the plunger (10x diameter = 100x surface/force).
If you close the pipe by a one direction shutter valve, you can repeat the pump movement and every stroke the big (100mm) plunger will move 1 mm. with 100 fold force.

Hydraulic systems work on the Pascal's Principle.
Pascal’s principle states that a pressure change occurring anywhere in a confined in-compressible fluid is transmitted throughout the fluid such that the same change occurs everywhere.
Hydraulic systems use an in-compressible liquid to transmit force from one location to another through the liquid medium.
Pascal's law can be stated as when there is an increase in pressure at any point in a confined fluid, there is an equal increase at every other point in the container.
Mathematically, We can write,
P1=P2 (since the pressures are equal throughout).
Since pressure equals force per unit area, then it follows that
F1A1=F2A2
It can be shown by substitution that the values shown above are correct, 1 pound / 1 square inches = 10 pounds / 10 square inches
Because the volume of fluid pushed down on the left side equals the volume of fluid that is lifted up on the right side, the following formula is also true.
V1=V2
by substitution,
A1×D1=A2×D2
A = cross sectional area
D = the distance moved or A1A2=D2D1
This system can be thought of as a simple machine (lever), since force is multiplied.The mechanical advantage can be found by rearranging terms in the above equation to
Mechanical Advantage(IMA) =D1D2 =A2A1
For the sample problem above, the IMA would be 10:1 (10 inches/ 1 inch or 10 square inches / 1 square inch).

The hydraulic press is composed of two hydraulic cylinders of different sizes, which are filled with water or oil. Water filling is called "hydraulic press"; Oil filled is called "oil press". There is a sliding piston in each of the two hydraulic cylinders. If a certain value of pressure is added to the small piston, according to Pascal's law, the small piston transmits this pressure to the large piston through the pressure of the liquid and pushes the large piston up. Let the cross-sectional area of the small piston be S1 and the downward pressure on the small piston be F1. Therefore, the small piston pair can be transmitted in all directions by the liquid with constant size ". The pressure on the large piston must also be equal to P. If the cross-sectional area of the large piston is S2, the upward pressure cross-sectional area generated by pressure P on the large piston is a multiple of the cross-sectional area of the small piston. From the above formula, if a small force is added to the small piston, a large force will be obtained on the large piston. In practice, a hydraulic press can be used to press plywood, extract oil, extract heavy objects, forge steel, etc.