=Domain, Domain name server (DNS) and DynDNS=
A DNS is important when you want to reach your server from internet because you don't want to learn your IP by heart, you'd rather buy a domain like mycompany.org and use it to reach your server.
# When you own a domain, you also own all the sub-domain like www.mycompany.org, erp.mycompany.org, yyy.xxx.mycompany.org ...; you can map this domain and its sub.domain to either an IP or another domain usually with the company that sold you the domain (there is other type of DNS record but we won't mention them here).
# In order to link you domain to an IP, you'll need an A DNS record for a IPv4 address or an AAA DNS record for a IPv6 address; in case you don't own a fixed IP you can use a Dynamic DNS[https://en.wikipedia.org/wiki/Dynamic_DNS] service: there is a daemon runing on your network that will keep updating your IP in the DNS (usually the internet modem/gateway have this funciton).
# In order to link your domain to another domain, you will need to setup a NAME DNS record (e.g. to the domain with an A record).
=Port Forwarding, DMZ and Reverse Proxy=
Once your Domain refers to your IP address, Internet requests will arrive to your internet gateway but you'll need to setup port forwarding[https://en.wikipedia.org/wiki/Port_forwarding] in order to forward those requests toward your web server.
The default internet port are 80 for http and 443 for https, once the
PF is configured it'll affect all the traffics coming on this port. If you have multiple web servers you may have to forward your traffic to a reverse proxy[https://en.wikipedia.org/wiki/Reverse_proxy] that will be able to redirect the traffic based on the url.
If you want to
avoid having internet traffic on your network (LAN), you can place the reverse proxy in a DMZ[https://en.wikipedia.org/wiki/DMZ_(computing)] with firewall rule that allow only the traffic from internet to the reverse proxy and from the reverse proxy to the local LAN (ie. your servers). For such setup there is an open source called pfSense solution that is able to do the firewall and the reverse proxy (with the module called HAProxy) .
your public IP can
=Let's encrypt / ACME= = DMZ and port forwarding= =Reverse proxy and SSL offloading=