Universal Two-Factor (or Universal 2nd Factor) devices are physical devices that offer a second level of authentication when accessing your online accounts. Generally, the first level of authentication is a password, but since passwords are relatively easy to steal, adding a second form of authentication makes it even more difficult for an attacker to compromise your account.
The second form of authentication can be a variety of things. It's common to use email or SMS, but these are some of the least secure options. Another common option is a one-time password (OTP) generator, like Google Authenticator.
One of the bests options for two-factor authentication is a U2F device. This is a physical device that can be used either via a USB connection or an NFC reader. A physical form of authentication is a good complement to a password because when used together, account access always requires something you know (password) and something you have (U2F device).