The Ranking is divided in Leagues. Each League is made up of 50 users, and, depending on your performance and the points you get, you can move up or down the League.
To move to the next League, you'll need to finish within the first 25 positions. You'll be able to visualize the position you need to be in to move up the League in the form of green boxes.
If you end up outside these boxes, in the red boxes instead, you'll go down. To access the Ranking go to the third icon on the inferior bar on the game's Main Screen.