A solute is a substance that can be dissolved in a fluid. One common example of a solute is ordinary table salt, or NaCl. Salt dissolves readily in water, which acts as the solvent.
Water is sometimes called the universal solvent. But not all solutes are compatible with all solvents, and water can only dissolve polar solutes. For example, water, which is polar, doesn't mix with oil, which is nonpolar. Salt is ionic, which means it is extremely polar and dissolves rapidly in water, alcohol and other polar solvents. Salt does not, however, dissolve in oil or chloroform, which are both nonpolar.