PTR record resolves an IP address to a fully-qualified domain name (FQDN) as an opposite to what A record does. PTR records are also called Reverse DNS records.

Here is how it looks like when performing external DNS lookup:

ptr record

PTR records are mainly used to check if the server name is actually associated with the IP address from where the connection was initiated.

There is one primary use for reverse DNS: when receiving mail, some mail servers verify the sending mail server has a PTR record configured for its address. This is a very simple check and is only used to ensure a PTR record exists and that the PTR record doesn't specify the name of a consumer internet service provider (ISP). Years ago, this check was a popular way to prevent spam from being sent from people's homes. Though most home ISPs now block outgoing SMTP so home computers can't send spam, a few mail servers still perform this reverse DNS check.

The PTR record for your server's IP address is controlled by the server provider, and you should contact your server provider to set the PTR record. The name used in the PTR doesn't need to be and generally isn't associated with any domains on the server. You do not need to change your server's hostname to match the PTR record. When you change your PTR record, there are no changes you should make to your server.