Read the short version
Human rounds use the published judging council. Read the current judging models.
On the head-to-head ladder the number is a rating. It moves on wins, draws and losses against real people, and beating stronger opponents moves it more. Your first three rated rounds are placement: the rating is live from round one, and the numbered rank arrives with the third. On the solo board the number is the judge's score for how well you argued.
Debated in tournaments before? Import your Tabroom record and start from a rating based on it instead of a blank 1500. Curious how the number works? The Debate Rating, explained.