What is a concentration gradient?

A concentration gradient is some region where the concentration of something varies from one end to the other. If you took a strip of paper and laid it on the floor and dropped pebbles onto it so that at one edge there were 2 pebbles per square inch, and at the other end there were 20 pebbles per square inch, and the number of pebbles per square inch increased as you went from the lowest- to the highest-concentration pebble edge, you would have a concentration gradient.