A concentration gradient is the process of particles or solutes moving through a solution from areas of greater particle concentration to areas of low particle concentration.
The importance of a concentration gradient is that it is what makes osmosis and diffusion to happen. If there were no gradient, there would be no movement of anything from high to low concentrations. Because cells are either hypertonic or hypotonic, nutrients and materials are able to flow either in or out of the cell and life is sustained. If all cells were isotonic, then there would be no flow of nutrients and materials and cells would not survive.