Severus Snape is one of the most important characters in the Harry Potter franchise. As the series began to delve into his past and how it shaped him, fans came to understand there was a lot more to Hogwarts' Potions Professor than met the eye. In Harry Potter and the Half-Blood Prince, Harry found himself in possession of an incredibly accurate potions textbook that brought him to the top of Professor Slughorn's class. Even Hermione was baffled by Harry's sudden success with potions, a class he'd never particularly excelled at before. Much to Hermione's relief, it wasn't Harry who seemed to have mastered potions. The book once belonged to a student who only referred to himself as the Half-Blood Prince.
Source：https://www.cbr.com/harry-potter-why-snape-half-blood-prince/
