A wire mesh filter is one of the most basic ways to filter impurities out of a liquid and also one of the most ancient. It is, however, a system that has never been abandoned since it was first developed. Modern technologies allow the creation of wire mesh filters to very tiny apertures. Though you may not think of them this way, bacteria and viruses actually do have a size to them. That can help you to understand, to an extent, how a wire mesh filter works.
One place you would be able to see a wire mesh filter in action would be at your local water treatment plant. A huge part of the treatment process involves sending the water through various sized screens to remove impurities. The largest screens take out the largest impurities. This is gathered up by the facility and either taken to a landfill and dumped or processed in some other way. As the water is run through tighter and tighter screen openings measured in microns, smaller and smaller particles are removed from the liquid. The size of the openings in modern wire mesh screens can be shockingly small.
Instead of imagining a very large filtration process, try to imagine a more precise process. Consider what chemists that work in the pharmaceutical industry do in the lab, for instance. These scientists need to be able to remove the smallest particles imaginable from the chemicals they work with. This is sometimes done to purify the liquid the particles are suspended in and sometimes it's done to get a sample of what is actually suspended in the liquid. The opening on the mesh filters these scientists use can be incredibly tiny, sometimes down to thousands of a mm or less. The companies that make these products have to have very precise manufacturing processes to make sure that the filters they sell are up to very high standards.
More Common Uses
A wire mesh filter is a component of some very common devices. For instance, you'll find them on the end of faucets and in fuel lines in gasoline engines. In these applications, their role is to remove particulates that could cause problems for the person drinking the water or for your vehicle. The wire mesh filter in these devices will have a pore size that is appropriate for the application. The end of your faucet, for example, doesn't have the same tiny openings that you'd see in a pharmaceutical lab.
In many cases, the wire mesh filter is part of a larger purification process. In the water treatment plant, for example, the water would usually be run through a process that involved UV light or some other bacteria-killing process before it was sent back into the environment. For thousands of years, however, wire mesh filters have been used in many different scenarios where particulate impurities have needed to be removed from liquids that were needed for drinking or other purposes.