Web, domains and email
RFC
Abbreviation for Request for Comments
In short and in plain language
An RFC is a public paper about tech rules for the internet.
A group that sets web standards writes these papers.
Many rules for email, domains and web addresses are in them.
If you set up email records, you mostly follow these papers.
Definition
An RFC is a publicly available document in which the Internet Engineering Task Force describes technical standards of the internet. Many rules for email, domains and web addresses are set out in such documents. Anyone setting up records such as SPF or DMARC usually follows the requirements in these documents.