The guide provides a comprehensive tutorial on using Firebase's legacy Realtime Database to perform basic CRUD (Create, Read, Update, Delete) operations via its REST API, using a sample database for an online retailer's internal app as an example. It highlights the process of creating data with defined keys using the PUT method, reading data with GET requests that can be filtered or ordered, updating data with PATCH to add or modify fields without overwriting nodes, and deleting records or specific fields with DELETE. The guide underscores the importance of data security by configuring access rules through Firebase's GUI or REST API, which can restrict read and write permissions using variables and conditionals based on authentication. It advises on best practices for structuring data in JSON tree format to enhance efficiency and security, such as avoiding nested data, flattening structures, and creating indexable data groups, all while emphasizing the flexibility and scalability of the Firebase Realtime Database.