For Adventurers League, the adventures are organized by character level groups. Tier 1 covers levels 1-4. Tier 2 is for characters 5-10, while Tier 3 is for characters of levels 11-16. There is also a Tier 4, which is for characters level 17-20. In order to maintain balance and prevent characters playing in adventures where they were seriously over- or underpowered, characters are only able to play in the tier that covers their level.
There are specific guidelines for creating an Adventurers League character, or there are pregen first level characters that will be available. Beyond that, you earn experience for each adventure you play, and characters must level up through play in order to advance to higher tiers. There are no pregens allowed past first level, although you may continue to play and level up a character you started with as a pregen.
I hope this answers your questions. If you have any more, ask and someone will be able to help you.
akammer wrote:Scott is absolutely right.