Difference with an 'e' was important for de Saussure and Derrida because the meaning of a term is given by difference between that sign and others. Words produce meanings because they are situated in a system of difference. Thus meaning needs other signs which are distant from the sign in question. So the meaning is not present but deferred, put off until that distance is crossed. But when you get there it's still deferred, because it consists in other still more distant signs, and so on.
Meaning is never present but always deferred.
Differance, with an 'a' means both to differ, as in difference with an 'e' and also to defer, or put off. Thus differance with an 'a' is ambiguous. So, Derrida says, differance is a non-concept, playing on the two meanings, forever suspended, never settled. Well, speech is supposed to be superior in communication than writing. Derrida wants to subvert that.