This is CS50 ID.
CS50 ID is an OpenID-based service that allows you to authenticate Harvard affiliates.
With CS50 ID, rather than ask users to log into your website (or even register), you can instead redirect them to CS50 ID, where they'll be prompted to log in with a Harvard email address or PIN. Once they've authenticated, CS50 ID will redirect users back to your website, at which point you'll know that they're indeed Harvard affiliates! CS50 ID will provide you with a unique identifier for each user (that you can use as a key in your own database). And, depending on how the user logged in, CS50 ID might also provide you with the user's full name and/or email address. CS50 ID will not provide you with users' passwords, HUIDs, XIDs, or PINs.
To add CS50 ID to your website, consult its HOWTO.