Retool is a next-generation WSIWYG (what you see is what you get) SaaS-based tool for quickly building distributed SQL and React applications from existing data sources, utilizing pre-built components and custom JavaScript. It supports over 30 native integrations, including PostgreSQL, AWS S3, and Cassandra, and is compatible with YugabyteDB, a PostgreSQL-compatible database. By following a step-by-step tutorial, users can install a YugabyteDB cluster on Google Kubernetes Platform, create a sample Northwind database, configure Retool to connect to the database, build a simple React application for managing inventory, test the application, and access it from mobile devices. The process allows developers to quickly deploy applications from their existing data sources, providing a fast and efficient way to build and manage internal applications.