What it is
Collects the DNS records that govern a domain's email. SPF lists which servers may send on its behalf. DKIM publishes the public key used to sign outgoing messages, so a recipient can tell nothing was altered in transit. DMARC ties the two together, telling receiving servers what to do when a message fails and where to send reports. BIMI, newer and far less common, attaches a verified logo to a passing DMARC policy so inboxes can display it.
Why it matters
A domain with no SPF, or a DMARC policy set to none, can be spoofed with very little effort, and most organisations find this out only after somebody has done it. The records also name the mail provider and every third party authorised to send, which is a reliable way to map an organisation's vendors.