DKIM adds a cryptographic signature to every email you send. Your mail provider signs each message with a private key, and you publish the matching public key in your DNS, under a name like google._domainkey.yourdomain.com.
Receiving servers use that public key to check the signature. If it matches, they know the message really came from your domain and wasn't changed in transit.
Gmail and Yahoo require DKIM for anyone sending in volume, and DMARC relies on it. For an outreach domain it's one of the first things to set up, before the first email goes out.
Example
Margin's outreach domain getmargin.example publishes a DKIM key at google._domainkey.getmargin.example, generated in its Google Workspace admin console.
Putting it into practice
Where to use it: SPF, DKIM and DMARC checker and Email deliverability for link outreach.