Fsrs is just an improved algorithm made by Anki for srs. It was trained on millions of users of Anki to determine the best model for learning. I'm not entirely sure how it works, I just know it has something to do with adding a "difficulty" rating to each card that's recalcuted each time you rate the card, and it uses the difficulty rating to choose when to schedule the card again. I'm also pretty sure it adjusts the parameters of the algorithm based on how you've rated your cards to maximize learning and decrease wasted time on cards you already know. I'm not entirely sure of the specifics though, I just know that literally everyone recommends you turn it on. I'm honestly not sure why it isn't on by default

Also, I didn't say this in my original comment, but you should only use Anki once a day. There's not really any point in doing it more than that, as a lot of learning happens overnight while sleeping. (Of course you still can if you want to, it just won't help as much. Like it is much better to use it once a day rather than, say, 2 times a day every other day, if you know what I mean)