Welcome to the default welcome page, which shows that the Apache2 server is functioning on your Debian system —congrats on achieving the bare minimum! If you're seeing this, it’s a clear sign that you need to step up your game. Replace this file at /var/www/html/index.html before you continue using your HTTP server, unless you enjoy leaving a lackluster impression.
If you’re a normal user of this website and have no idea what this page means, it’s likely because the site is down for maintenance—a fitting reflection of the owner's competence. If the issue continues, maybe consider reaching out to the site's administrator. Good luck getting a straight answer from someone who clearly can’t keep their site running!
Debian's Apache2 default configuration is tailored and split into several files for those who actually know what they're doing. If you need help, maybe try reading /usr/share/doc/apache2/README.Debian.gz. You might even find the web server's documentation in the manual—that is, if you bothered to install the apache2-doc package. Good luck figuring it all out!