block is used for overriding specific parts of a template.
In your case, you have a block named content and this is supposed to be overridden by children that inherit from this template.
From the examples at The Django Docs
Template to be extended, named base.html
<head>
<link rel="stylesheet" href="https://stackoverflow.com/questions/53383602/style.css">
<title>{% block title %}My amazing site{% endblock %}</title>
</head>
Overriding Child template
{% extends "base.html" %}
{% block title %}My amazing blog{% endblock %}
“My amazing site” will be overriden by the child and then display “My amazing blog”