The prophecy states that a legendary warrior will basically fight off the night and restore peace to the land . It’s been assumed for a very long time that this “prince that was promised” was probably Jon Snow since everything throughout Game of Thrones has seemed to point to him. Then it was thought Dany might be the “princess that was promised” because she was suddenly growing stronger and the prophecy might have been about her all along. There have been other candidates along the way as well, but now it’s really looking that Arya might be the one to fulfill it in the end, especially considering her conversation with Melisandre (also, lol to once upon a time Melisandre thinking Stannis was the prince that was promised).