It stands for 'your name' and comes from fanfiction. People write self insert fanfics where the reader is one of the main characters and y/n is where you'd imagine/insert your own name
The reason why you'd see many comments here saying this has 'y/n x ceo' vibes is because that's a major trope found in fanfics. Because of the nature of the y/n trope, the y/n character is usually written as a very generalizable character who many ppl could seem themselves as (a more blank slate personality for example or a personality that revolves around the male lead). Although I don't usually read y/n fanfics, I'm enjoying reading this story with all its cutesy vibes and love! Hope this helps :D
What is y/n ?