Krishna and Radharani are the male and female aspects of the Supreme Being, appearing together. Krishna is One, but He appears as two so that there can be dynamic exchange of love. It is difficult to comprehend the relationship Krishna and Radharani through mental speculation, as Krishna is the very source of the mind, and cannot be understood with our limited human intellect.
If Radharani is both one with and different from Krishna, you might ask can two people be one, and one be two? The combination is like that of sun and sunshine. There is no sunshine without the sun and the sun cannot exist without the sunshine. The energy (sunshine) and the energetic (sun) are one, while also being different. Similarly, Radha and Krishna are simultaneously one and different. Krishna is the Supreme Personality of Godhead, while Srimati Radharani is His supreme pleasure energy. As one, they constitute the Absolute Truth.
In his commentary on Srimad-Bhagavatam, Srila Prabhupada notes that the relationship of Krishna and Radharani is not a romantic dealing between two mortal human beings. It is purely transcendental and requires spiritual qualification to correct comprehension. Srila Prabhupada recommends that an aspiring devotee of Krishna should first thoroughly study the initial nine cantos of Bhagavatam —all of which stress upon the greatness of God and His divine energy— before delving into the more intimate pastimes detailed in the Tenth Canto.
Radharani makes her prominent presence felt in the Hare Krishna mantra. Hara is the feminine form of Hari or Krishna. And "Hare" is a vocative form of "Hara." Radharani is highly compassionate and for this reason her name appears before that of Krishna as devotees approach Her before approaching Krishna.