double curly brace {{

In my understanding, these braces are used to pass python variables to the HTML template. I will give you a simple use case here: For example, I want to pass the value of a variable first_name. In the views.py:

def my_html_template(request):
   first_name = 'John Doe'
   
   return render(request, 'my_html_template_name.html', {'first_name': first_name})

And then in the 'my_html_template_name.html', use the dictionary key to retrieve the value:

<p> {{ first_name }} </p>

This will evaluate to:

<p> John Doe </p>

The curly braces are part of Django Template Language

The part encapsulated between double curly braces {{ }} is nothing but a variable.

That's how DTL, Jinja2 and other template languages work. They have their own set of rules which translates the template in to python and later to HTML code.