I think this is when the Doctor finally comes to terms with River’s death, the guilt over what happened at the Library, and his feelings for her. He knows he can see her again if he chooses, because she implies it. Spoilers. Yes, that can be that River is letting the Doctor know that Clara is alive. But, I think it’s beyond that as well.
I think this is when the Doctor becomes River’s Doctor. Because he’s accepted his love for her and her death. He can go to younger her and freely love her and be the Doctor that River knows and have the marriage that River remembers and treasures and tells Ten about in “Forest of the Dead.” It’s not over for them. I fully believe she’ll be there when he regenerates.
It’s called the TARDIS. It can travel anywhere in time and space. And it’s mine.
“It’s called the TARDIS. Short for Time And Relative Dimension In Space.”