The complete conception of God includes God and His energies, which exist simultaneously with Him and are in a sense identical to Him. The sum of Krishna's energies is Radha, His eternal consort. Like the sun and sunshine, Krishna and Radha are one.
Radha embodies the totality of love for Krishna. Thus She is known as the personification of Bhakti, or loving devotion to God.
Loving God is our natural state, now dormant. By Her pure example, Radha is the beacon calling everyone to re-awaken full love for Krishna. She also dispenses mercy to help us achieve that goal.
Radha-Krishna are the male and female aspects of God. Known as the Divine Couple, together they are the full manifestation of God.
"Radharani is a tenderhearted feminine counterpart of the supreme whole, resembling the perfectional stage of the worldly feminine nature. Therefore, the mercy of Radharani is available very readily to the sincere devotees, and once She recommends such a devotee to Lord Krishna, the Lord at once accepts the devotee's admittance into His association." —Srila Prabhupada, Bhagavatam 2.3.23