Force-carriers are particles that are responsible for the action of forces, such as electromagnetism. Just as photons are responsible for electromagnetic force, so mesons are responsible for some of the lower energy ("residual") strong force interactions that occur between nucleons. (Strong force is also known as nuclear force or "residual strong force" when its action is between nucleons.) At an even smaller level, gluons are responsible for the strong force interactions between quarks. 