Sand filters are fairly simple in their design. In a standard cartridge filter, the water flows into the filter through the inlet port. It then passes through a diffuser at the top of the filter that distributes the water evenly into the top of the sand bed.
As the water passes down through the sand, the debris particles in the water are trapped between the particles of sand. As the sand gets older, it becomes less efficient at filtering out debris particles.
Once the water reaches the bottom of the filter, it is collected in the lateral assembly at the bottom of the filter. From the lateral assembly, the water passes out through the outlet pipe.
The filter has an air bleed tube inside that will bleed off almost all the air that builds up inside the filter. There is also a manual air bleed on the top of the filter that can be used to bleed off the air or to let air into the tank when you are preparing to take the filter part (after you have turned the pump off).
There is a pressure gauge on the top of the filter. When installing the filter, or when putting it back together after cleaning, you should make sure that the face of this gauge is visible to you when you are turning the equipment on and off so you can see any dangerous rise in pressure.