Answer 8 questions about your skills and experience. Kèo suggests a starting rating so you're matched with equals from your very first match, instead of "grinding" up from a common baseline. After that, your rating adjusts to real match results.
A self-assessed rating only applies while you have fewer than 3 confirmed matches in that event, and it stays within a set range (nobody can rate themselves straight into the top tier). Overrate yourself and losses will cost you more.