What is MFA? (multi-factor authentication)
MFA stands for multi-factor authentication: signing in requires the password plus at least one more factor, typically an authenticator-app code, a push approval or a hardware key. It exists because passwords leak constantly, and it turns a stolen password from a breach into a failed login.
Not all second factors are equal
SMS codes beat nothing but fall to SIM swapping; push approvals fall to prompt-bombing when users tap approve to make the phone shut up; authenticator codes are solid; FIDO2 hardware keys and passkeys also resist phishing, because the browser will not release a credential to the wrong site. Ranked, roughly: passkeys and hardware keys, then app codes, then push, then SMS.
Where it matters most
Email accounts (they reset everything else), anything internet-facing (VPN, remote access, the RMM itself), and admin accounts everywhere. An MSP's own tooling is a bigger target than any single client.
Related terms: EDR, zero trust.