[Answered ]-I want to display 'yyy' in the index.html, but when running the server, the page displays normally but without the 'yyy'

try this

you need pass dict into template from view.

https://docs.djangoproject.com/en/4.2/ref/templates/api/#django.template.Context

def index(request):
   context = {'a1': 'yyy'}
   return render(request, "index.html", context)

in template

<h1>{{a1}}</h1>