Flask-Bootstrap packages Bootstrap into an extension that mostly consists of a blueprint named 'bootstrap'. It can also create links to serve Bootstrap from a CDN and works with no boilerplate code in your application.
Here is an example:
from flask.ext.bootstrap import Bootstrap [...] Bootstrap(app)
This makes some new templates available, containing blank pages that include all bootstrap resources, and have predefined blocks where you can put your content.
As of version 3, Flask-Bootstrap has a proper documentation, which you can check for more details.