What is DNS?
DNS turns a name like example.com into the address of the server.
What it is
DNS, the Domain Name System, is the phone book of the internet. A browser cannot do anything with example.com until something says which machine sits behind it. A DNS record turns the name into an address.
The name and the address are two things. While the record is missing or points somewhere else, nobody reaches the app, even if it is running.
What it is made of
There are several kinds of records. An A record points at an IPv4 address, an AAAA record at IPv6. A CNAME points at another name instead of storing an address itself. MX records belong to email and say which server accepts mail for the domain. For a website or an API you usually need the record that places the name on your server, and often another one for www.
What goes wrong
Every record has a TTL, a span of time for how long other servers remember the answer. A change is therefore not visible everywhere at the same moment. If the name points at the wrong server, people land on an old or foreign site. If the record is missing, the site does not arrive at all. If the name does not match the certificate, the browser warns, even when the server itself is running.
What you otherwise look after yourself
The domain and these records are part of the setup you otherwise repeat for every project. The application code does not change that. Two projects on the same server still need their own names, or at least their own records.
How it shows up on MainPath
Under Domains MainPath shows the records the name needs and checks whether they are set at the provider. Once the record and the certificate are in place, the app is reachable under that name.