What does a concentration gradient do to etc?

Concentration of a chemical in a solution refers to how many of the chemical's molecules are sitting in a small volume of the solution. Concentration could be measured in molecules per liter, although molecules are so small compared to a liter that we usually use different units (just like we wouldn't want to measure the distance between the earth and the sun in inches). A gradient is a measurement of how much something changes as you move from one region to another. So a concentration gradient is a measurement of how the concentration of something changes from one place to another.

If this doesn't help here's a Khan Academy video https://www.khanacademy.org/...and.../concentration-gradients