Once your character is at level 45, you will receive an invitation from the guild in your inbox. The invitation will start you on a quest which will unlock Undaunted Pledges. Pledges are daily quests which task you with defeating certain bosses within a unique dungeon.
For successfully completing a heist, you can earn gold, experience, Thieves Guild reputation, and containers of valuable goods that can be sold or kept for the Covetous Countess" quests.