Reddit mentions of Radical Atheism: Derrida and the Time of Life (Meridian: Crossing Aesthetics)

Honestly, this is really, really hard to explain without a lot of context and a fair bit of background knowledge of Husserl, Heidegger, and probably some Kant. To my mind the best, clearest, and most persuasive interpreter of Derrida right now is Martin Hägglund, so I would recommend that you check out some his work. This essay on Derrida and Levinas is a good place to start (see especially 42ff.), though ultimately the most detailed account of this stuff you're going to get is in his book, Radical Atheism.

The short answer is yes, you are missing the point somewhat. Différance isn't just about words and meanings; it's about time and temporal succession. What concepts like différance and 'the trace' are about is a is a way of describing the logical relations obtaining among the elements of a temporal synthesis through which distinct temporal moments are put in relation. This synthesis acts as a condition for the possibility of identity over time (X is the same X at T1, T2... Tn), but it also undermines a picture of identity that Derrida--following Heidegger--believes dominates the history of philosophy (where there is some atemporal essence or property of a thing by virtue of which it remains identical over time). For Derrida, as it were it's time--not turtles--all the way down; there are no atemporal essences, properties, beings, meanings, etc., etc.

When Derrida says that différance is neither a word nor a concept, he's speaking in a way that I find somewhat annoying. I think the point that he is trying to make is that to talk about a condition for the possibility of identity over time as something like a word or concept would be a category mistake: words and concepts (and meanings, for that matter) are conditioned by time and are thus conditioned by différance. The "neither a word nor a concept" line seems like a silly way to make that point if you ask me, but people more sympathetic to Derrida's writing style than I am might want to make a case for that way of putting it. Différance isn't a word or concept just in the sense that it is meant to be describing a transcendental condition for the possibility of words and concepts.

Really, though, I would strongly recommend checking out the Hägglund and seeing whether that helps. It's just hard to know where to begin when it comes to giving a more substantive definition of différance.