The 'Witches' Rune', a chant originating from Gardnerian Wicca, contains the line 'Queen of heaven, queen of hell,' referring to the Wiccan Goddess.
At first glance, the reference to heaven and hell may appear disconcertingly Christian to modern pagans. But the concepts are of course much older and more universal than the Christian concepts. Personally, I interpret the phrase as meaning 'Goddess who is both astral and chthonic'.
I don't take 'heaven' here to refer to an afterlife, but the literal 'heavens'. The stars and the moon have always been symbols of mystery, magic and inspiration. I should emphasise that I don't understand the Goddess as a symbol of the stars and moon (they are, after all, merely balls of gas or rock), but rather vice versa - the stars and moon are symbols of the Goddess.
'Chthonic' refers to that which is in or related to the earth. This includes the underworld (i.e. 'hell'), an almost universal pagan concept, and underworld deities. Interpreting myth through psychology, it's hardly a big leap to associate the underworld with the unconscious mind.
There are probably as many understandings of the Goddess as there are Wiccans (and other Goddess-worshipping neopagans). It is not an easy topic to put into precise words, and I'm not claiming to be any kind of authority on her, either.
She is often described as a mother or creator deity. But I think I prefer to think of her as the goddess of creativity, in a more artistic sense, and of imagination. She is a muse, or the Muse. This could perhaps be seen as part of the heavenly, or astral/lunar, aspect.
(In the ancient world, creation was considered a gift from the gods and not a personal achievement. In the modern world we of course associate it strongly with individuals, but then again we are also more likely to view deities as part of our subjective experience of the world, as archetypes, so there is really no contradiction between these two views.)
Then again, it is easy to associate her with dreams and intuition, as well as darker impulses of the unconscious. These could be seen as part of the chthonic aspect.
There is considerable overlap between these two aspects, of course. They are not meant to be in any way clear-cut. Particularly her close relationship with magic and ritual can perhaps be seen to draw from both the heavenly and chthonic aspects.