A device key is simply a code required to access the Review Solution app. This is what ties an instance of an app on any of your devices to your business. When you sign up for an account, you’ll get an automatic “Welcome” email that includes your device keys and dashboard login information. It’s ok if you don’t keep the email, though; your device keys will be in your app dashboard so you can access them anytime.
If you forgot your device key, just open the app, scroll down a bit, and tap "Forgot device key?" After you verify your account's email address and phone number, you'll receive another copy of your "Welcome" email, which includes all your device keys.