Derrida refers mostly to the binaries of presence/absence, being/nothingness, and speech/writing. But in explicating the deconstruction of those binaries, he (and others) deconstruct other binaries such as man/woman, nature/culture, truth/error, good/evil, identity/difference, self/other, and unity/fragment.
We start with the binary speech/writing. Note that Derrida shows how in the history of Western philosophy, and in the way that history is reflected in our language, the element of a binary which is seemingly more present is given priority over the other element. Speech seems more present than writing because it is immediate to the speaker and therefore the meaning or truth of what is spoken seems more spatially and temporally present to the speaker's Being (the word most commonly used to describe existence).
However, beginning with Saussure's description of the sign as signified and signifier, we see that speech is also marked with an absence. A meaning is divided into word (signifier) and idea (signified). The word itself, "tree," is spoken but its signified idea is not there in the word; it is an abstract idea. Just as writing is missing its author, speech is missing its signified meaning. In each side of the binary, there is an absence of some kind. Thus, each side of the binary (writing and speech, which is to say all language) requires something else (a supplement) to make meaning. Thus, the meaning of each word, written or spoken, requires some other thing and therefore the meaning of any word must defer to some other different words/meanings. Differance means to defer and to differ.
By the definition of a definition, you need other words to describe a word. So, the meaning of that one word/signified must defer to those different words/signifieds in order to mean something. The word "tree" is arbitrary. We could use any word to represent the idea of a tree. We need the relationship of the word "tree" (and the relationship of the signified meaning) to other words (and other signifieds) in order to mean something.
Since differance sounds like difference in French, we need the written version of differance to get the meaning. The very word Derrida uses to explain language and the bias of the binary requires writing to get its meaning. Here is an easy example which shows that speech is not necessarily more present or true/original in terms of meaning.
In Freudian analysis, woman is marked by an absence of the phallus (psychologically and biologically). However, a deconstructor would say that in defining a man, we need to explain how he is not a woman. Therefore, man is marked by the absence of a woman. Furthermore, this is to say that man is marked by the absence of an absence: the absence (woman) of an absence (woman without a phallus). This shows the folly of bias of privileging so-called presence and the reversal of the binary through showing difference:
The first consequence to be drawn from this is that the signified concept is never present in and of itself, in a sufficient presence that would refer only to itself.
In differance, speech defers to writing, man to woman, and vice versa. Neither is more present or more original.
Derrida does not show mere reversal; he shows how there is no sense in prioritizing one side of a binary (in terms of presence) over another because every element is marked by absence; every element defers to differences in order to mean something.