Flask is licensed under a three clause BSD License. It basically means: do whatever you want with it as long as the copyright in Flask sticks around, the conditions are not modified and the disclaimer is present. Furthermore you must not use the names of the authors to promote derivatives of the software without written consent.
The full license text can be found below (Flask License). For the documentation and artwork different licenses apply.
General License Definitions¶
The following section contains the full license texts for Flask and the documentation.