Amazon Cognito is a service that makes it easy for developers to handle user identity, not just within the application itself, but across multiple devices. Amazon Cognito allows you to focus on building your app by handling all of the complexities related to signing up new users, remembering user details and keeping sessions alive between multiple devices.
If you've ever had to build an authentication/user management system before, you'll understand how much time and effort this would save. I had to recently use terraform to deploy this for our client.
Cognito User Pools allow developers to easily add user sign-up and sign-in capabilities to their mobile or web applications without having to build back end support for managing users or dealing with issues like session replication, cross-device syncing and password resets. In addition, Amazon Cognito Sync allows app developers to store application data in a central location so that it can be accessed by multiple different client devices.