Authentication is the act of verifying who a user is, and is the basis of the login process. Typically, authentication uses the combination of an identifier--a username or email address--and a password. The user submits these values through a form, and the application then compares the submitted information against that previously stored (e.g., upon registration).
In Yii, this entire process is performed semi-automatically, leaving the developer to merely implement [[yii\web\IdentityInterface]], the most important class in the authentication system. Typically, implementation of `IdentityInterface` is accomplished using the `User` model.
Two of the outlined methods are simple: `findIdentity` is provided with an ID value and returns a model instance associated with that ID. The `getId` method returns the ID itself.
Two of the other methods--`getAuthKey` and `validateAuthKey`--are used to provide extra security to the "remember me" cookie. The `getAuthKey` method should return a string that is unique for each user. You can create reliably create a unique string using `Security::generateRandomKey()`. It's a good idea to also save this as part of the user's record:
The `validateAuthKey` method just needs to compare the `$authKey` variable, passed as parameter (itself retrieved from a cookie), with the value fetched from database.