In nature, a liquid that has a higher solute concentration passes through a permeable membrane towards a liquid with a lower solute concentration. The permeation will cease once the two liquids achieve equilibrium (that is, they have the same amount of solute). This, in a nutshell, is osmosis.
Reverse osmosis flips the process on its head by applying pressure on the liquid that has a higher concentration of solute (including bacteria) so that it passes through a superfine filter (membrane) towards a tank with clean water. According to AXEON Water Technologies, a reverse osmosis water filter system removes up to 96.5% of total dissolved solids, resulting in pure water that even people with weakened immune systems can safely drink.