You have heard this before from deSausssure. He worked through the concept of the sign, he argued that relationship between signifier and signified is arbitrary, and showed that signs only mean in relation to each other (remember "muton" and "sheep"?) Derrida quotes deSaussure, who wrote "in language there are only differences. Even more important: a difference generally implies positive terms between which the difference is set up; but in language there are only differences without positive terms." Derrida re-emphasizes the point that meaning isn't in the signifier itself, but that it only exists in a network, in relation to other things. Différance comes before being. This throws the idea of "origin," of true original meaning, into radical question (pp. 114-5).
2. At the heart of existence is not "essence" but différance.
With the idea of origin in question, Derrida pushes further than deSaussure did to claim that there is no absolute identity, nothing that "is itself" by virtue of its being. Thus transhistorical truth is now only truth by virtue of difference: nothing stands outside the system of differences. Derrida encourages us to think of this in terms of "play," by which he means both a kind of game where winning and losing happen in turn (as opposed to thinking this problem as the absolute loss of meaning) AND as something which has some "play" in it, like an "articulated" joint, another favorite Derridean term (pp 122-3).
3. Différance calls into question time and space.
The header of this section should serve as a warning: this stuff is dense. Différance is a word Derrida made up to point out the following:
c) It comes from both "defer," to put off, which is what happens to meaning in language in a relational system where signs themselves do not have essential meanings, and from "differ," to be unlike, not identical. The "ance" gerund form of the word puts it somewhere between passive and active, like "resonance" or "dissonance" (113).
d) He joins the sense of différance as time (defer) and différance as space (unlike, distinction between proximate things) to make the point that an idea of "being" and/or "presence" that has been so central to Western culture is not so authoritative. If meaning is not "present" within the sign, and if signs instead signify in relation to one another only, haven't we been hanging on to an idea of presence-as-authoritative meaning that has led us to privilege speech over writing? He turns "presence" into a philosophical security blanket (114). The instability of "presence" as being extends to consciousness, which is the idea of self-presence (119).
Although we don't get a terribly good definition of the trace in this section, it is important to Derrida's thought and will be important in Lacan as well. On p. 116, he discusses the trace as both the mark of the future and the past in a present moment which is neither. The idea of our present (a meaning-full present) depends on this trace, which is an effect of writing. On p. 121, he asserts that the concept of the trace is inseparable from the concept of difference. He also refers (defers?) to Freud's definition of the trace as an effort of life to protect itself by deferring the dangerous investment, by constituting a reserve. We will discuss this phenomenon in the context of psychoanalysis at greater length. Derrida tackles Heidegger's use of the trace on p. 126 and, as he does with deSaussure, pushes it further to suggest that the trace is the "essence of Being" that haunts language.