When you have a force that is applied on a surface over a given surface area then you say that a pressure is being exerted on the surface.
Fig 1 shows a small wooden block placed on a wooden sheet. As you know every object has a weight. This weight as you would remember is a force of attraction that the earth is exerting on the object. Thus it is a downward force.
Hence we can deduce that block of wood due to its weight is applying a force F on the sheet. The force F would act as shown in fig 2 below.
But we also know that the weight of the object, the force applied on the wooden sheet, is being applied over a surface area A. That surface area is shaded black in fig 3 below.
The pressure exerted on the wooden sheet by the wooden blockis thus the force acting perpendicularly per unit surface area.
Hence to calculate the pressure we have to use the equation below:
Pressure exerted = Force applied / surface area
The unit of pressure is thus the N m-2