Explain what a concentration gradient is and how it assists a cell in doing its

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.  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. In the presence of a concentration gradient, many molecules can't move across the cellular membrane. Large molecules cannot fit through the double layer of phospholipids, charged molecules are repelled, and hydrophilic molecules have trouble crossing the hydrophobic interior of the membrane. This often results in higher concentrations of chemicals on one side of the membrane than on the other.

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As queen28 mentioned, many molecules cannot move across the cell membrane because they are too large or they are not permeable. However, even if they are not able to, molecules always want to move down their concentration gradient, from high concentration to low concentration. If the molecule is able to move across a membrane by passive diffusion it requires no work, so concentration gradients can be exploited to move molecules from one area another assuming that the molecules are permeable to the membrane.

A concentration gradient is the differnce in the number of molecules present in two different regions.e.g a rasien which is dipped in water sweels up because of the concentration difference. The rasien has less concentration of the solvent as compared to the solute n thus water from the surroundings enter in the resin and it swells.
This differnce in the concentration is helpful for the cell as it helps the cell to maintain a constant flow of ions in n out of the cell. This process is also responsible in the production of ATP in the cell.