But what exactly is pressure? Thinking about particles

What is pressure?

Here the macroscopic phenomenon of pressure is related to the actions of the particles.

Fluids – liquids and gases – consist of many small particles in constant random motion. These particles exert forces on any surfaces that they happen to collide with. The force arising from the pressure is therefore due to particle bombardment on surfaces. The pressure is a measure of the state of the fluid: the pressure in the fluid. One thing this measure can predict is the force that will be acting on an area bounding the fluid.

Notice that the direction of the force is set by the orientation of the area. The force acts on the area because of the differential bombardment by the particles inside and out (so the pressure difference). We think its helpful to talk about the pressure in the fluid and the force acting on the area.

It is the bombardment of water molecules on your ears that creates the force on your eardrums. If the water pressure is big (because you dive down to the bottom of the ocean) the collisions of the water molecules on your eardrums are more frequent and a greater force is generated.

We need to picture the action of pressure (always in fluids) as being due to particles colliding with surfaces. Decreasing the volume leads to more collisions, therefore a greater pressure.