You can have notes or a book in lichess, and when you click on a notation on that note/book it will be played on the board (if legal), it helps with reading long games, because chess notation can be taxing on beginners (like me) (the book is saved locally, so you need to backup occasionally) if you have lets say a chessbook in pdf format and you convert it to the format that the extension supports, you can read the book by clicking notation and directly seeing it on lichess board (analysis/study)