While Flask doesn't force us to use any particular templating language, it assumes that we're going to use Jinja. Most of the developers in the Flask community use Jinja, and I recommend that you do the same. There are a few extensions that have been written to let us use other templating languages, like Flask-Genshi and Flask-Mako. Stick with the default unless you have a good reason to use something else. Not knowing the Jinja syntax yet is not a good reason! You'll save yourself a lot of time and headache.
Note Almost all resources imply Jinja2 when they refer to "Jinja." There was a Jinja1, but we won't be dealing with it here. When you see Jinja, we're talking about this: http://jinja.pocoo.org/