It's an application server. If you're familiar with the Python webdev ecosystem, Passenger is roughly a Ruby equivalent for something like uWSGI or Gunicorn. (IIRC, you can actually run Python apps with Passenger now, too)
Where now = for 5 years or more.
Something like mod_perl / mod_xxx in the early days... and they did some work on the concurrency problems with rails and ruby around the ruby 1.8 days iirc... In the meantime they expanded to more languages...
Not sure if you mean this page or not:
https://www.phusionpassenger.com/
But as they say, it's an application server. Similar to mod_perl for apache/php, unicorn or indeed nodejs (which can function as an application server for Javascript).
It’s a Ruby web server that can we bed used with nginx or Apache.