Heidegger was an important influence on Derrida. Heidegger's concept of destruction led to Derrida's philosophy of deconstruction and both philosophers questioned the nature of "presence."
Heidegger's notion of Being, in terms of "Dasein" ("being there") and the human "being-in-the-world" does not separate the human being's mental activity from the external world. The human being is always already necessarily a part of (not apart from) the world. Therefore, Heidegger does not ascribe to Descartes' idea that the mind and world are separate entities. Heidegger also says that, in being connected to the world ("being there" and "being-in-the-world"), human beings are communal beings, being present to each other.
Heidegger sought to investigate the meaning of "Being" because he thought this question had been forgotten throughout the history of philosophy. Considering all the notions of Being in human experience and Being in terms of time, Heidegger asked how and why Being has been described in terms of "presence." This question inevitably led to considerations of time. The question of Being seemed to be stuck on this notion of being present. But presence had always been thought of as a mode of time and Being is timeless. (If Being is being, then it always IS.)
Derrida's term "differance" also deals with meaning in general and concepts of time. The word "differance" literally means to defer and to differ. Differance is the function or movement of meaning. For example, a mouse is a small mammal, a mouse is not a house, a mouse has a tail, and so on. We know what a word (like mouse) means by its relationship to other words and in a context. We could also say an X rhymes with house, is a small animal, and we'd eventually determine that X means "mouse." So, it is not the word that defines the meaning; it is the relation of the word/idea to other words/ideas.
Therefore, a word has meaning because of differance. The word defers to other different words; this deferral is a spatial and temporal movement. Thus, the meaning of the word "mouse" is never fully "present" with the word mouse by itself because it must defer to other different words. This deferral "happens" over space and time. Derrida's notion of differance, how meaning works, shows that meaning is never fully present.
Heidegger questioned the meaning of Being as it had always been linked to presence (in life for human beings, or timelessly as a metaphysical entity or idea). But Derrida shows that meaning of anything (from the "mouse" to "Being" itself) is never fully present. Therefore, even Being can not be defined solely in terms of its "present-ness."
Therefore, it is the determination of Being as presence or as beingness that is interrogated by the thought of differance.
Indeed, differance challenges any notion of absolute presence in Being, in meaning, and in the meaning of Being.