ASP.NET Core Identity is a membership system that lets you add user accounts to your ASP.NET Core applications. It provides the low-level services for creating users, verifying passwords and signing users in to your application, as well as additional features such as two-factor authentication (2FA) and account lockout after too many failed attempts to login.
When users register on an application, they typically provide an email/username and a password. ASP.NET Core Identity lets you provide validation rules for the password, to try and prevent users from using passwords that are too simple.