The "5000" in the name implies scale—a massive library of titles ranging from retro arcade classics to modern .io games. While the original domain has changed hands over the years due to legal pressures, the keyword persists as a "search flag" for students looking for the latest mirror site.
If the main site is blocked at your school, look for "Mirror" links or alternative sites like Unblocked Games WTF 1v1.LOL - unblocked games 5000 Save Your Progress: