For most of us, when we say "Divine Mother" or "The Goddess" we are talking about the Supreme Being as the creatrix of the universe who expresses compassion and love in a very personal, direct, and intimate way in the same way that a loving mother cares for her children.
Words fail to say who or what the Goddess is but She can be known and experienced in a direct and personal way. This means that we can't say for sure how big She is or where She came from or what Her ultimate form is or why She is. What we can do is enter into a relationship with Her, a communion that is very intimate, close, loving and personal. She will be our best friend and confidant and our patient teacher.
Both the creation stories in Genesis and in the Hindu Rig Veda begin with the idea of the formless, the void, and the absolute. Then creation arose (or is continually arising) out of this. The Rig Veda describes this "arising" as a divine whim or play of the Supreme. The creation that occurs is Mother (maya), the divine whim (Shakti or power) is also Mother and the absolute formless ground of being is, again, Mother. We can grab hold of Her skirt at any point along this timeless expression of the Divine and, if we hold on tightly, She will dispel our illusions and deliver us to the blissful and peaceful realization of Divine Truth.
Maya, in Her potential aspect, is the divine power of the Lord. She has no beginning. She is composed of the three universal qualities (gunas), subtle, beyond perception. It is from the effects She produces that Her existence is inferred by the wise. It is She who gives birth to the whole universe. - Adi Shankara, The Crest Jewel of Discrimination Translated by Swami Prabhavananda & Christopher Isherwood
When the Supreme Being is thought of as actionless - neither creating, sustaining nor destroying - I call Him by the name of Brahman (pure absolute formless being) or Purusha. But when I think of him as active - creating, sustaining and destroying - I call him by the name of Shakti (Divine Mother as primal energy) or Maya (the veiling power of the Divine Mother by which the One appears as many) or Prakriti (Divine Mother as primordial nature which, in association with Purusha, creates the universe).
The actionless Brahman and the active Shakti are in fact one and the same. He, who is the Absolute Existence-Intelligence-Bliss, is also the All-knowing, the All-intelligent and All-blissful Mother of the universe. A precious stone and its luminosity are one and the same, for you cannot imagine a diamond without it, and vice versa.
God the absolute (formless) and God the personal (Divine Mother; form) are one and the same. A belief in one implies a belief in the other. Fire cannot be thought of apart from its burning power; nor can its burning power be thought of apart from it. Thus God the absolute cannot be thought of apart from the idea of God with attributes (Mother). - Ramakrishna - The Gospel of Sri Ramakrishna Translated by Swami Nikhilananda