Ranked matches are 90-second rounds against players near your rating. Each player you finish above counts as a win and each one above you as a loss, and beating a stronger player moves your rating more. The tiers go from Rookie through Bronze, Silver and Gold to Diamond.
If no match turns up after 12 seconds of searching, you can race runs recorded by players rated near you instead, with Gumbots in any empty places. Only your rating moves in those races.
Ranked opens once you have finished three games.