In his On Grammatology, Derrida shows that there is a binary opposition between speech and writing—with speech being central and natural, and writing being marginal and unnatural. People have been writing for thousands of years, but the tradition of thought from ancient times to modernity favors speech, the spoken word, over the written word.
The “logos,” which the ancients regarded as the hidden principle in the universe, is a voice or a word—it is a sort of super-word or a God. When we speak, the sounds that we create are closer to the super-word or God, but our writing is marked by an absence, the absence of the Godly sound. What Derrida is really going after is Saussure’s linguistics.
Saussure had asserted that his linguistics was free of the viewpoints related to God. Derrida shows that Saussure privileges speech because he accepts the pre-scientific assumption of speech being closer to inner meaning, or the logos, and the super-word and God.