1. A mesh water filter at the inlet pipe traps big bits of dirt
2. Smaller impurities like rust and sand are removed by a sediment filter
3. An activated carbon water filter adsorbs organic matter and contaminants
4. The chilled filtered water is zapped by a UV purifier keeping it free from bacteria
5. Whilst the boiling water is treated to prevent scale (those yucky white flakes).
Unless you’re getting your water direct from a borehole or well most of this is taken care of by the water company before it gets to your tap.
edit: to be clear it’s essentially an expensive descaler (point 5)