From the D&D Adventurers League website:
If you like a deeper dive into a "convention-style" campaign experience, then D&D Expeditions is for you. Set in the Moonsea region of the Forgotten Realms, you'll explore the storyline as it affects the denizens of that area of Faerûn. Here, you'll get to take your character from its humble beginnings to the heights of high level play, and have a chance to really influence your region through your character's actions and the actions of your faction.
Adventures are delivered digitally as PDFs to stores and conventions requesting them, and debut at selected conventions around North America throughout the storyline. If you play in a debut of an adventure, you'll have the opportunity to give feedback and shape the ongoing storyline. In addition, these adventures will be supported with kits that contain the certificates for the magic items and other special stuff that characters can earn through play.
We expect that D&D Expeditions will become the public's D&D campaign, growing and changing as events resolve and new threats emerge. Each storyline visits a different area of the Moonsea, affecting some change there, and all the factions are involved. For Tyranny of Dragons, we'll be setting adventures in and around the town of Phlan, an old fan-favorite from the days of Pool of Radiance.