In our treatment of fluids here, we will assume that they are incompressible . That is, the volume of a quantity of fluid does no change with pressure. This assumption is quite valid for water. We will use it with air as well, and this is a very poor assumption for large variations in pressure. However, we will apply it to air with care.
Pressure is related to force, but it describes the distribution of force over surface area.
In words, pressure is the force per unit area. The pressure of a fluid describes the force per unit area that it exerts on the walls of its container and on objects submerged in it. Just as the tension in a massless string is constant along its length, in the absence of frictional forces and so forth, the pressure in a volume of massless fluid is the same at all points within it. We can't get very far with the assumption that fluids are massless. In fact, in the presence of gravity, pressure varies with depth in a fluid.