A water filter is a mechanical device used to remove foreign substances out of water. It works like a sieve with holes of varying size. In some cases only solids are filtered out. This means that substances which are solved in the water stay in there.
If there is a membrane as filter (for example for reverse osmosis) also solved particles are removed and the result is fresh water.
Filters are also used in private homes directly at the tapping point. We use this in our office to remove impurities from the building’s piping system.
Caution: A filter needs to be cleaned or exchanged regularly. Otherwise it might block the flow of water.