This is a webserver that can run various web applications for you. In a typical setup you'd run a web/http server (nginx or Apache etc) to handle the 'slow' connections to the client and a separate app server (say, an express application, or Django or Rails etc) that is completely separate from the webserver. The web server acts as a reverse proxy, sending the request on to the app server. The response from the app server is then passed to the webserver, which sends it back tot he client. So a request would this path: client -> web server -> app server -> web server -> client.

This is done because web servers are optimised to handle (many) requests from clients (including things like TLS termination, optimised handling of static files, orchestrate to which backend service to send a request and much more), while application servers are optimised to run the application code.

In this model, the web and app servers run separately, for example in separate containers and they have to talk to each other so you have to somehow connect them (often this is done through a network socket).

So what Nginx Unit does is combining the two: it can do the standard web server stuff (not everything a normal nginx can do, but often that's not needed anyway), but also run your application for you, which could, depending on your setup, make your life a bit easier, as now you don't have to tie the two together.