The first power to come into being was Chaos. Then arose Gaia,
broad-bosomed earth, which serves as the ever-immovable base for
all the immortals who dwell on the peaks of snowy Olympos;
and then shadowy Tartaros deep in the wide-wayed earth;
and then Eros surpassing every immortal in beauty,
who, a loosener of limbs, brings all immortals and mortals
under his power and makes them unable to think as they should.
And out of Chaos black Night and Erebos came into being,
and out of Night then came the brightness of Aither and Day,
whom she conceived by lying in love and mingling with Erebos.
The first power was Chaos. Then came Gaia, the wide earth, which is the base for all immortals who live on Olympus. Then dark Tartaros deep inside the earth. Then Eros, the most beautiful, who rules all immortals and mortals at some point, stopping them from thinking with their brains*. From Chaos, Night personified and Erebos became. From Night, Aither and Day, after making love with Erebos.
*Eros is where we get the word "Erotic", and he is lust personified, hence he rules both the gods and men at some point. (you know it's hard to think when you have a crush on someone. blame Eros.)
The great chasm of Chaos is the starting point from which Theog. moves to the cosmos of Zeus's orderly government of the world. Night and Erebos arise out of Chaos, and so apparantly do Gaia (with Tartaros) and Eros. Eros has a place among the earliest powers because of his role as the generating force in creation.
Hesiod usually describes all the divinities of one generation before moving on to the next. Thus he gives us Gaia and Night here, then the Children of Gaia (126-210) and the Children of Night (211-32), and finally the Grandchildren of Gaia (233-500). This scheme, however, is broken in the case of certain divinities who are better listed with the first mention of their parents. Thus Aither (the brightness of the upper air) and Day are listed here along with their parents and necessary complements Erebos (the darkness below) and Night. We might note that Sun is not born until later (371) and that therefore, as in Genesis 1, day is not connected with the light of the sun.