Very simply a water filter is something which cleans water. It removes things from the water that can cause problems, both for your health and your plumbing.
If you have your own well, then there are lots of things you would not wish to have in the water you drink, or your washing.
Even when you have piped water from your local water company there can be lots of contaminants. Especially things like chlorine, which are used to keep water free from bacteria. Having bacteria in your water is bad, having chlorine in your water is bad too.
These are just the common things.
There are a great many things which are invisible.
And undetectable, without special equipment. Lots of unpleasant, and unhealthy, chemicals can be in your water. A whole house water filter like the AquaOx is how you get rid of them. With better grades of water filter removing more of the unhealthy chemicals and problems, that affect your life and your health.