Internet Identity is an innovation that was researched and developed by the DFINITY Foundation. Internet Identity, allows you to use a singular log-in across the open web. Any service built on the Internet Computer can take advantage of this cryptographic innovation that allows users to log in without the need for a password, seed phrase or any personal identifying information. Instead, you create authentication profiles that use the authentication methods you choose such as facial recognition from a smartphone, your computer unlock password, or a security key.
The Internet Identity service enables you to authenticate securely and anonymously when you access applications that use the service as an authentication method. A different identity is created for each application you log in to, and you will be able to use all of your registered devices or authentication methods to log in to the same account.
For FAQs on using Internet Identity of the NNS Application (or other applications which use Internet Identity) please refer to this FAQ: https://dfinity.org/faq/internet-identity-and-nns-front-end-dapp-faqs