Web, domains and email
RFC
Abbreviation for Request for Comments
In short and in plain language
An RFC is a tech paper about the internet.
It sets out rules and standards for the net.
Many rules for email and domains are written down in such papers.
Definition
An RFC is a technical document that describes rules and standards of the internet. Many procedures around email and domains, such as SPF or DMARC, are specified in such documents.