Osmosis refers to the passage of water thru a thin membrane from the side with low salt concentration to the side with higher salt concentration. This can happen even when the water level is higher on the high salt side and the water must move against a pressure difference. The bottom line is that osmosis refers to a concentration difference manifesting itself as a pressure difference. The other side of the coin is reverse osmosis. A difference in pressure is used to cause a difference in salt concentration.
Salt means any inorganic compound dissolved in water.
When water is processed by reverse osmosis a large fraction of dissolved material is removed. The cleaner the input water the cleaner the output water will be. Conversely, if your input water is clean enough, you may not need an RO unit. If they are fed well water, microbes will build up and clog the membrane. TFC membranes are used with well water as they resist the clogging.