Definition
Three DNS records that prove a cold email actually came from your domain and wasn't altered in transit, and tell inboxes what to do if that check fails.
Without them, a receiving mail server has no way to confirm a message is genuine, so it defaults to suspicion, which is why cold email without proper authentication so often lands in spam regardless of how good the copy is.
Where this comes from
Read: Cold Email Deliverability: Why Your Emails Land in Spam (And How to Fix It)