Where does the csrf_token() function come from in flask_wtf.csrf?

Question

I've implemented a flask app and needed to use a CSRF token. The code looks something like this:

<form method="post">
    <input type="hidden" name="csrf_token" value="{{ csrf_token() }}"/>
</form>

The jinja templating uses the csrf_token() function, which I assumed was coming from Python. However, the function isn't imported or initialized anywhere, so I was wondering if maybe it's initialized when the CSRFProtect function is run? It looks like this when we import and initialize things:

from flask import Flask
from flask_wtf.csrf import CSRFProtect

app = Flask(__name__)
csrf = CSRFProtect(app)

The docs are here: https://flask-wtf.readthedocs.io/en/0.15.x/csrf/