N/L
Means 'Your last name.' Used in fanfic when you're too lazy to write your whole name and just want people to know who you are.
In the fanfic, the main character said 'Y/L/N is Smith.'
She wrote 'Y/L/N: Johnson' and the reader was confused.
He typed 'Y/L/N: Lee' and it was the best part of the story.