“Shore” is a measure of the hardness of a material, named after Albert F. Shore. The letter designates the “scale,” which specifies what type of substance is described. In this case, A refers to flexible mold rubbers/plastics.
The numbers go from 0 to 100, with 0 being the softest and 100 being the hardest. Hence, cap septa with a 55 Shore A rating are harder than those with 45 Shore A.