Two-factor authentication is a second step that bolsters the strength of all those long, random passwords you’re now using. This security measure is based on the notion that you need two items to prove your identity: Something you know and something you have. The "something you know" is your regular password, and the "something you have" is usually a short, one-time code generated via text massage or smartphone app that you need to enter before logging in.
Two-factor authentication is not foolproof, but it’s far stronger than using passwords alone. Google’s Authenticator app for Android, iOS, and Blackberry 4.5-7.0 is a popular choice for generating one-time codes, but there’s also the cloud-sync service Authy for Android, iOS, Linux, Mac, and Windows.