A solution is a mixture of two or more  substances. It has a solvent and a solute. Depending on the concentrations there are different kinds of solutions and their applications differ too. Isotonic solutions have the same salt concentration as the surrounding blood cells. So an isotonic solution will have no effect on the surrounding cells. The cells will not gain or lose water if place in isotonic solution.  The normal Saline solution which is 0.9% NaCl is isotonic with blood and is used in patients suffering from dehydration or if they can't take any liquids or food.