Amplification means increase. Differential means that some mutations are increased more than others. Beneficial mutations are increased in the next generation. Neutral mutations are neither increased nor decreased. Deleterious mutations are decreased in the next generation.
It works like compound interest. Beneficial mutations have a small positive interest rate. Neutral mutations have a zero interest rate. Deleterious mutations have a negative interest rate, ranging -100% for fatal mutations (no offspring) to a small negative interest rate for mildly deleterious mutations.