What is nginx

The nginx package is a web server — Apache httpd and Microsoft IIS are other examples of web servers — and that is likely the web page that opened when Safari launched.

That could be your default start page, for instance.

But whatever web page was accessed wasn't online when you reached it.

In isolation, this can be perfectly innocent (except for the folks managing the web server and the web page you reached, if there was supposed to be content displayed), or it may indicate that there are other concerns. Check your start page, disable plug-ins, review what's been installed, etc.