When someone has shared something with you, you will receive both an email notification and an in-app notification. A jewel beside Activity in the left side bar will let you know that you have an unread notification.

Unread Notification Jewel

To view your Notifications, click on the unread notification jewel or open the Activity View by selecting Activity in the left side bar.
Notifications are sorted to the top of the Activity view and are identified with an X in the right hand column.

Notifications Identified with an X
You can view the document being shared with you by clicking its link.
You can dismiss the Notification by clicking the X in the right-hand column.