DKIM stands for DomainKeys Identified Mail. It's an email authentication protocol, using digital signature attached to the header of email. In a nutshell, when DKIM is published in the DNS records of a domain, it will display a public key that the recipient email servers can match with the encrypted key in the signature.
As for any email authentication protocols, having DKIM properly set improves the deliverability of emails sent from this domain. Indeed, some email servers can block the access to email with no DKIM record published.
In the same way, your domain has less chances to be blacklisted or have a bad SpamAssassin grade if it has a DKIM record published.
In a nutshell, having DKIM properly setup will make your emails more secure while increasing your deliverability.
Spam filters are using keyword detection to spot junk emails and make sure your inbox stays clean. This is why you should choose carefully the words used in the subject and body of your emails. Here's is the ultimate list of spam-trigger keyword that you should ban from your email content in 2021!