What exactly is "server_name" in nginx configuration file ?

This question was answered by @olavamjelde:

Each server block in your /etc/nginx/sites-enabled/default will have a server_name.

Server name is what it will listen for, together with your port settings. So let’s say you have a domain, you point your A name like: example.com to your droplets IP. This lets the DNS server know where the domain should go (like a street adress).

Then you can set a server block inside the default file, with server name: example.com But you wish to also have this accessible via www.example.com, that mens you change it like so: server_name example.com www.example.com;

If you then want to have a subdomain like: yolo.example.com and have it work in another directory, you make another server block and you there add server name: yolo.example.com

You also have to add the a name in the DNS agian, point yolo.example.com to your ip. For maximum flexibility use: a name>floating ip>your server

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