This prompt seems to be pretty recent as beta users only noticed it a couple of nights ago, so like we said it sounds like Apple is really pushing for users to secure their accounts as best as possible. Clicking on the notification will take users to the Settings app where it explains to users what the feature is about, and how to enable it.
For those who are unfamiliar with two-factor authentication, basically this is where in addition to you entering your username and password, an additional layer of security is applied where you will have to enter an additional code once your username and password has been verified. This can come in the form of a one-time generated code sent to your phone.
There are some banks that also provide dongles to their customers where these codes can be generated as well. This means that in the event of someone guessing your password, they’ll still not be able to login unless they have the code, thus making your account a bit more secure.