DNS turns a domain name into an IP which can then be used to send data through your router, a dns server is the server which is used to do this conversion (www.google.com turns into an IP 1.2.3.4 (that isn’t the actual IP of google)).
There are many dns servers, normally your local devices use your router as the dns server, which forwards it to your ISP which they further transfer it over global dns servers.
Alternatively you could use Google’s DNS server (8.8.8.8) or cloudflares DNS server (1.1.1.1) but if the one on your router works then just use it.
nameserver is the same as DNS server
Tldr: set the router IP as your dns server, you only need this one.
One thing that makes a project good is knowing what it does, I’ve seen quite a few projects where they talk about all the features and technology and how to configure it but not a word about what it actually does, what problems it solves and so on.
I won’t self host your program if you don’t even tell me what it does, don’t make me search and clue together large parts of the documentation just to find if I want it. A simple explanation is enough but somehow I’ve seen quite a few programs that don’t have it.