The Couchbase Lite Viewer is a tool designed to allow developers to inspect and manage documents within Couchbase Lite databases, particularly useful when developing iOS or Android applications. It enables users to browse existing documents, insert new ones, and display soft deletions, which is demonstrated through a todo list application example. The screencast covers steps like installing the Couchbase Lite Viewer, using the App Browser to open a database on iOS, making modifications in the app to see real-time updates in the Viewer, and locating and copying the .cblite file on Android using ADB commands to open it in the Viewer. Additionally, it includes instructions for Android users on navigating the filesystem of an emulator or device using ADB shell commands and copying files to the host filesystem.