Who is Radha Rani? We chant her name every day, Radhey Krishn or Radha Krishn during ‘naam-sankeertan.’
Radha and Krishn both took avatar around 5000 years ago, in a place called Braj in India, where they spent their time doing leelas, walking with their lotus feet on this very earth. Even today, if you go to the Braj district, in towns like Nandagaon and Gokul, where Shree Krishn grew up, and in towns like Vrindaban, Mathura, Barsana, and Govardhan, where they performed several leelas – in all of these places everybody still greets you with with the name of God, ‘Radhey Radhey.’  Everybody says Jai Shree Radhey. Even if you want someone to move out of your way, you’ll say Radhey Radhey to get their attention!  
What is the importance of Shree Radhey? 
Who is Radha Rani? 
Simply put (Ye Yam Radha Yasch Krishan),
Vedas tell us that Radha is Krishn. Shree Krishn is purnatam purushottam brahm.
Brahm means absolute supreme God. His own self is Radha. He Himself appears as male form Shree Krishn and female form as Radha Rani.  You can say that Radha herself has become Shree Krishn, or Shree Krishn Himself has become Radha.
They are not two separate personalities. They are one and the same. One supreme Divine power appearing in two forms.
Yayam Radha Yasch Krishno……
Vedas tell us that in order to do playful leelas, to reveal those playful past times, Radha Krishn although being eternally one, they eternally exist in two forms. One Divine personality in two forms.
Shree Radha Rani is supreme Goddess. She herself is Shree Krishn. This is the easiest way to understand who Shree Radha is.
Radhey Radhey!
Please also see a short Youtube video by Swami Nikhilanand Ji on the question “Who is Radha?” .