By AppID we mean the unique identifier that is associated per each one of your apps, on Bright SDK dashboard. Once you add a new app, as explained here, it generates a unique AppID, that will be used to count every active device and ensure it is associated with your account, and that you are being paid for it.
This AppID is used as part of your implementation for Windows and Roku. To learn more, follow the corresponding integration guide.
AppID is not used explicitly when integrating iOS/tvOS, Android and WebOS. Instead, it is based on your app's ID in the relevant app store.