What is a PTR record? Do I need to create one?

PTR records resolve IP addresses into hostnames.

For example, the A record for subdomain.exemple.com resolves this domain name to 123.123.123.123.

And then the PTR record resolves 123.123.123.123 back to subdomain.exemple.com.

This is called reverse DNS.

PTR records aren't created for domain names and your domain name can't have one to make sure your mail will not be rejected by other mail servers.
Indeed, PTR records are created for IP addresses, not domain names.

If you are using some other provider's server and that mail server IP does not have a PTR record created, we have no way to change that and you need to contact the company which owns that mail server. For example, if you are sending mail through your ISP mail server (it's always the case if your are using a client mail like Outlook), you will need to contact them and request to configure PTR records for all IP addresses their mail servers are using.