What is the "base" file that the Bokeh index.html extends?

Question

I am a little confused about what {% extends base %} is extending at the start of index.html in Bokeh server application packages.

Examples of this can be seen in:

• Bokeh Docs: Embedding in Templates

{% extends base %}

{% block contents %}
<div>
    <p> Hello {{ user_id }}, AKA '{{ last_name }}, {{ first_name }}'! </p>
</div>
{% endblock %}

• Bokeh Server Application Examples
  Example code from the Gapminder package in templates/index.html
  

{% extends base %}

{% block title %}Bokeh Gapminder Example{% endblock %}

{% block postamble %}
  <style>
    {% include 'styles.css' %}
  </style>
{% endblock %}

What is this "base" that is being extended?

I see that there is a "contents" block, "title" block, and "postamble" block from the above examples.
How do I know what other jinja blocks I can modify?

Thanks.