Using a Yubikey is part of a process known as 'Two Factor Authentication' which combines two things; something you know, which would be your password, with something you have, which would be the Yubikey.Rather than replacing the standard login, it adds to it. You need to enter your username and passphrase as you normally would, as well as activate the Yubikey to send a 'One Time Pad' single-use code to be verified in order to sign in. If you don't know the password, or don't have the Yubikey in your possession, you can't sign in, significantly reducing the chances of unauthorized access to your account.