Release Jaheira from her cell is a small quest in Baldur's Gate II: Shadows of Amn. Jaheira, like Minsc, is stuck in a cage at the beginning of Baldur's Gate II: Shadows of Amn and needs to get out to find her husband.
Jaheira is location northeast of the PC and Imoen. In order to get her out, she needs a key. The key is in the northwest room where you get your initial equipment. Grab the key from the table and use it to unlock the cell keeping Jaheira captive. You will be given an option for Jaheria to join you.