Pressure is caused by forces acting on a surface. The greater the force or the smaller the surface area, the greater the pressure produced.
It's the old drawing pin example. You push the flat end with your thumb to spread out the force. The sharp end enters the wall as the smaller area concentrates the force, producing a higher pressure.
We can calculate pressure using:
P = pressure (N/m2 or Pa, Pascals) - Note: 1 Pa = 1 N/m2
F = force (N)
A = surface area m2
Solids can transmit forces easily. If you push one end of a metal bar the other end will push what ever is near to it in the same direction. Liquids and gases can't do this.
Pressure is useful because it can be used to transmit forces from one place to another using liquids and gases.
Stage 1: 10 N force down on small area.
Stage 2: Liquid under pressure, where .
Stage 3: Same pressure now exists at the large area, 100 N/m2.
Stage 4: So an upward force is produced, where F = PA = 100 x 1 = 100 N.
You can see from the example above that a 10 N force is moved from the left side to the right side, and is also increased by using a larger area.
Note: Energy is can't be created from nowhere, so the small area will have to move down much further than the larger area moves up!
This idea is used in hydraulic systems - for example, car brakes. A person puts a small force onto the foot pedal, which creates a large force on the car brakes pads.