Water solubility refers to the ability of a substance to dissolve in water. When a solute, such as salt or sugar, is added to water, it breaks down into smaller particles and spreads throughout the liquid. This process creates a solution, where the solute is uniformly distributed within the solvent, which in this case is water. The degree of water solubility varies among different substances. Some compounds, like sodium chloride (table salt), are highly soluble, while others, like oil, are not soluble at all. Factors such as temperature and pressure can also influence how well a substance dissolves in water.