Tiers are used to measure your progress in the game. You must have a set number of Trophies in order to move into each Tier, but remember, you can also drop back to lower Tiers if your trophy count falls below the Tier's requirement. To see your current Tier and information about other Tiers, tap the Seasons button on the main menu, which is highlighted in green below.
Moving into a new Tier unlocks one or more new Tracks to race on and improves the rewards you receive from Chests!