Srimati Radharani is the Supreme Goddess. She is most always seen with Lord Krishna. It is described that She is the Chief Associate and devotee of Lord Krishna, and topmost of all Goddesses. Her name means the She is the most excellent worshiper of Lord Krishna. However, She is also an expansion of the Lord’s energy. Since She is also an extension of Krishna, She is the feminine aspect of God. Thus, in the Gaudiya Vaishnava tradition, God is both male and female. They are One, but Krishna expands into two, Himself and Radharani, for the sake of divine loving pastimes. If They remained as One, then there is no relationship, there are no pastimes, and there can be no dynamic exchange of love. (Caitanya-caritamrita, Adi-lila, 4.55-56) Actually, if we all remained merged or amalgamated into one single force or light, then there is no further need of anything else. There certainly would be no need for the material manifestation to provide the innumerable conditioned souls with the means to seek out the way to satisfy their senses, minds, emotions, desires for self-expression, intellectual pursuits, and on and on.
So, similarly, the spiritual world is the manifestation wherein all souls have the opportunity to engage in a multitude of pastimes in loving relationships in full spiritual variety, without the many hindrances we find in this material world. The only difference is that the spiritual world is centered around the Supreme Being. And that Supreme Personality has expanded Himself into Radharani for exhibiting the supreme loving relationship, in which so many others assist Them.
In the Brihad-Gautamiya Tantra, Radharani is described as follows:
devi krishna-mayi prokta
kantih sammohini para
"The transcendental goddess Srimati Radharani is the direct counterpart of Lord Sri Krishna. She is the central figure for all the goddesses of fortune. She possesses all the attractiveness to attract the all-attractive Personality of Godhead. She is the primeval internal potency of the Lord."
To explain further, Srimati Radharani is also the source of the other goddesses, who are expansions of Her. Just as Lord Krishna is the source of all other expansions and incarnations of God, Radharani is the source of all other expansions of the energies of God, the shaktis, or other goddesses. Thus, Vishnu, Rama, even Shiva are all expansions of the one Supreme Being, and similarly Lakshmi, Sita, and even Durga are all expansions of this Supreme Feminine form of God, Radharani.