The Great Thaw was a magically-induced shift in climatic conditions that marked the end of Elsa's winter through the restoration of summer to Arendelle. Following the return of summer, Elsa allowed the castle gates to remain open, and Arendelle was once again open to the outside world, ending its 13-year-long isolation.
Late during Elsa's winter, a massive blizzard ravaged Arendelle after the queen was brought back to the kingdom following the assault on her ice palace. While running through the storm, Elsa encountered Hans, who deceived her with news of her sister Anna's death. Devastated by the news, Elsa was able to stop the storm, suspending the snow in the air with her grief.
Hans then moved in to kill Elsa as she mourned, but Anna managed to intercept the blow, freezing to solid ice simultaneously. Though Anna appeared lifeless, her act of shielding Elsa was an act of true love, which caused her icy form to thaw out.
Thawing of Arendelle
With the realization that "love will thaw", Elsa saw the key to controlling her powers and reversed the winter weather plaguing Arendelle. She gathered the ice and snow covering the landscape into a massive snowflake and cast the contents into the sky. With Elsa finally able to control her powers, she and Anna finally rekindle their sisterly bond.
Following the end of winter, those who had visited Arendelle for Elsa's coronation were able to depart. Elsa resumed her duties as queen, cutting ties with Weselton and granting Kristoff the title of Arendelle Ice Master and Deliverer. She also opened up the castle so that the citizens of Arendelle could skate in the courtyard, and vowed never to close the gates again.