While it is a fluffy marketing sentence, it seems clear to me. It's an application server.
Then the next section explains that you can run different kinds of applications and even different versions of the base software like different versions of PHP or python all in the same server.
> NGINX Unit is a new, lightweight, open source application server built to meet the demands of today’s dynamic and distributed applications.
That means everything and nothing. They wasted their intro sentence on it. They probably don't have a very good idea of what it is either.