Reimu is getting overly stressed from the burdens of being a cornerstone of the hakurei border that she and Yukari are bearing. Yukari takes away that part so she's just a regular girl taking on all of the responsibility of the border on herself.
Ran is worried it will be to much for her alone without the shrine maidens help.
It seems a bit of time goes on and Reimu is living a normal life when during a dream she remembers a memory of Yukari and her when she was still a shrine maiden. She remembers because even though she has less worries, she has forgotten all the good and bad memories that came with being a shrine maiden.
She then has Yukari give her back her self which she had hidden because it's better to live and experience everything. Yukari warns her all the burdens will come back in full force which is where she says they should share that burden. When she wakes she is back to her normal self as she greets yukari.
I'm sure it could be better worded, but I tried.
tl;dr
Reimu got super stressed, Yukari made her forget the bad parts of life. She took a break then got her memories back and everything is back to normal.
