It’s a story about how Applebloom feels horrible after Twilight was turned to stone by the cockatrice back in the episode “Stare Master”, but then later saved by Fluttershy.
She feels that if Fluttershy wasn’t able to save Twilight, she could have been lost forever. Thus making her feel extremely sorry.
I suggest reading the story! I don’t want to spoil the whole thing. It has a lot of feels in it, so be prepared!
Here’s the story—–>www.fimfiction.net/story/14918