Chaos is the primordial state of the universe, also referred to as the primordial being in Greek cosmology. Back then, everything was Chaos and Chaos was everything. All other concepts and or beings were non-existent.
At some point (we are unclear on the how, as no partner is specified), five beings followed from this state of distortion. First, there was Night (Nyx) and Darkness (Erebrus), who covered the Universe in a pitch-black fog. Then there was Earth (Gaia) and the Underworld (Tartarus). Finally, there was Love (Eros).
Although not specified in a literal sense, Chaos is replaced by order later on. This is what happened when the Titans and (later) Gods took over the reign of the universe.