How are ratings calculated, and what does provisional mean?

Ratings use Glicko-2. Everyone starts at 1500 ± 350: the number is how strong you look, and the ± is how sure the system is. Win and your rating goes up; play more and the ± shrinks. A rating is provisional while its ± is above 110, which means few rated games so far, a recent change, or a long break.