Why is there a question mark (?) next to a rating?

The question mark means the rating is provisional. Reasons include:

• The player has not yet finished enough rated games against opponents of similar strength in the rating category.
• The player hasn’t played enough recent games. Depending on the number of games you’ve played, it might take around a year of inactivity for your rating to become provisional again.

Concretely, it means that the Glicko-2 deviation is greater than 110. The deviation is the level of confidence the system has in the rating. The lower the deviation, the more stable a rating is.