RAM, even though it generally refers to the integrated circuits that store memory in a randomly-accessible form, can include many different components. These components have different access times. For instance, your hard drive is sometimes used, in part, as RAM. The hard drive, however, will generally have a slower access time than the integrated circuits that provide the bulk of the heavy lifting for the computer’s RAM. The hierarchy refers to the concept applied to in computer architecture that prioritizes the components with the fastest access times being used for RAM. This makes your computer run faster, to put it in the simplest possible terms.