Two-factor authentication (2FA) is a security method that requires two different types of identity proof before granting access to an account, so a stolen password alone is not enough to get in.
2FA requires two proofs of identity from different categories: something you know such as a password, something you have such as a phone or a hardware security key, and something you are such as a fingerprint. Because the two proofs come from different categories, compromising one does not hand over the other.
After you enter your password as the first factor, the service asks for a second factor before signing you in. Common second factors are a code sent by text message, a time-based code from an authenticator app, a push notification you approve, or a hardware security key. Access is granted only when both factors check out.
Text-message codes are the most convenient but the weakest, because they can be intercepted through SIM swapping. Authenticator-app codes and hardware security keys are stronger, since they are not sent over the phone network and stay on your device.
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