Her whole stalker situation. Look up “Billie eilish stalker letters” on google and read what this dude said. So scary! The line “I go back to the diner, I write another letter. I hope you read it this time. You better.” Refers to those letters

https://www.reddit.com/r/billieeilish/comments/1ctxpg0/song_discussion_thread_the_diner/