Your Backup Kit allows you to safely store your Master Password, New Device Key and account email address. If you ever lose access to all of your logged in devices or log out and uninstall the application, you'll need all of these to get back in.
You should be walked through creating a Backup Kit when you first create your account. If you were not, or if you skipped this step originally, you can generate a Backup Kit from within the RememBear app.
Here's how to manually create a Backup Kit:
Head into your settings, and locate the 'Account' tab. There, you'll see the option to "Get Backup Kit".
Head into your menu by clicking the three horizontal bars, select 'Settings' and click "Create a Backup Kit".
Whether you print your Backup Kit or write down the details by hand, be sure to store it somewhere safe. You'll need these three pieces of information (Master Password, New Device Key, and Account Email address) if you ever lose access to your account.