In this context, these symbols are conventional signs taken to refer to meanings that are not themselves. Concepts are apprehensions of form, or we might say "meanings", the what-it-is-to-be of the given thing. When I write "123", that series of characters obviously is not itself the number 123. A human interpreter in possession of a mental dictionary can read that series of squiggles to arrive mentally at the concept of the number 123. But all that exists in computers are representations, not the things they are meant to represent, and their meanings are entirely in the mind of the human observer who assigns conventional meanings to those representations.