The concept of difference comes from the structuralist Ferdinand de Saussure. He claimed that there was no inherent relationship between a word (spoken/written) and its signified idea or object. Consider that there are many languages and therefore many different words for tree. The letters that form "tree" mean tree because we as a community agree that "tree" represents tree. But "tree" has other forms in other languages. It is not the word that creates meaning; it is the difference of the word (signifier) "tree" to other words that gives it meaning. The word "tree" is a signifier and meaning is established by the relationships of difference "tree" has with other signifiers and thereby with other signifieds (ideas/objects). "Tree" means something only when it has a context: a tree is a plant that makes its own food that takes in carbon dioxide, etc. (We literally need other words/ideas (signifiers/signifieds) in order to explain "tree." "Tree" can not mean something on its own.
Derrida made up the word "différance" to further explain this notion that meaning is derived from difference. "Différance" means to differ and to defer. Thus, the meaning of the word "tree" requires other signifiers/signifieds for meaning to be established. So, the meaning of "tree" differs to those other words and in a real temporal deferral, the meaning of "tree" is (time-wise) deferred to other signifiers (such as forest, water, plant, etc.). And theoretically speaking, we could go on forever describing what a "tree" is, looking up more and more definitions to describe the ones we've already read. For example, a tree is not an animal. It is a plant that lives in a forest which is not a city which has automobiles which emit pollution which can harm the environment of which trees are a part of, and so on.
Now, for the deconstructive move. The history of Western philosophy has always privileged speech over writing because speech seems more present, closer to the person's mind, soul, etc. This history has linked speech, origin, truth, and Being with presence. If something is less present, it is less original, less true, and further from existence (Being). Derrida will turn this privilege of presence on its head.
Derrida used "différance" because in French, it sounds the same as difference. Therefore, in speech (the supposed more present and truthful medium of communication) there is no difference between difference and différance. Only when written do we see the difference between the two words. And since meaning is derived from difference, it seems that writing is at least a supplement to speech or, in cases like this one, writing is more equipped to establish meaning than speech is.
This is an example of how a binary opposition (speech/writing) can be reversed. "Tree" needs other words to mean something. The spoken word différance needs its written form to mean what it means. Thus, the meaning of "tree" (or of the spoken word "différance") has meaning only by moving from its present form and by differing from and deferring to other things. So, it is not presence (of a word, idea, or event) that conveys meaning; it is the movement from that presence to things different that conveys meaning.
Try to describe orange without using other words or colors. Différance is the movement from a presence (orange) to things different, a theoretically endless movement.