DKIM is a feature that allows emails to be digitally signed, so their authenticity can be verified by a recipient. This helps stop spam and phishing emails as they’re often faked/spoofed, which is something DKIM is designed to stop.
If you have a dedicated email hosting service, it’s already enabled. However, you may need to take some extra steps to get it working.
If you want to enable DKIM on your cPanel service, you can do so under cPanel > Email Deliverability.