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Osmosis is the movement of water through a semi-permeable membrane and is actually a form of diffusion. The movement takes place across water potential gradient, i.e., from a region of high water potential to a region of low water potential. Water potential is a measure of concentration of free water molecules (the ones that are free to move), in other words, the potential energy. Water potential of pure water is zero (in kPa or kilo Pascals) and that of a given sample, containing solutes, is a negative number. When solutes are present in water, water molecules tend to react with them or surround solute molecules and hence are less free to move (and thus the negative potential). This also means that more the solute concentration, lesser would be the tendency of water molecules to diffuse. Osmotic potential is also known as solute potential and is a measure of water potential for movement from a region of lower solute concentration to higher solute concentration. Osmotic potential decreases with an increase in solute concentration (since water molecules will be less free to move due to more solute molecules).
Plants make use of solute potential to ensure delivery of water molecules to roots and leaves.
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