Yii::$app->session is a component in the Yii framework that provides access to the session data for the current user. The Yii::$app variable represents the application instance, and by accessing the session property, you can interact with the session data.

For example, you can use Yii::$app->session->get('key') to retrieve a value from the session using the specified key. Similarly, you can use Yii::$app->session->set('key', 'value') to store a value in the session with the specified key.

To check if a key exists in the session, you can use the has() method like this: Yii::$app->session->has('key'). This will return a boolean value indicating whether the key exists in the session.

To remove a value from the session, you can use the remove() method: Yii::$app->session->remove('key'). This will remove the value associated with the specified key from the session.

You can also flash data to the session using the setFlash() method. Flash data is used to store temporary data that can be accessed in the next request and then automatically removed from the session. For example, you can use Yii::$app->session->setFlash('success', 'Message') to set a flash message with the key 'success' and the value 'Message'.

In addition to these basic operations, Yii::$app->session provides other methods for managing session data, such as getting and setting multiple values at once, checking if the session is active, and destroying the session.

It's important to note that Yii::$app->session relies on PHP's session handling mechanism, so you need to make sure that session handling is properly configured in your PHP environment.

Overall, Yii::$app->session is a convenient way to work with session data in the Yii framework, allowing you to store and retrieve data specific to each user session.