Two-factor authentication, usually shortened to 2FA, is a login protection that asks for two separate proofs that you are who you say you are. The first is something you know, your password. The second is something you have, usually a code from an app on your phone or a text message. The point is that a password on its own is no longer enough to break into an account. Passwords leak constantly, whether reused across sites, caught in data breaches, or guessed by software that tries thousands a second, and 2FA means that even when someone has your password they still cannot log in without the second code. For any WordPress site with more than one user, and especially for a membership site or anything handling client data, I treat 2FA on every account as a basic requirement rather than an upgrade. It is one of the cheapest, highest-value security steps you can take, and turning it on takes a few minutes. You have almost certainly used it already: the code your bank texts you before it lets you in is two-factor authentication.
Glossary entry