Flask SQLAlchemy is a powerful tool for creating web applications with relational databases. It provides an easy-to-use interface for working with databases in Flask applications, leveraging the strengths of Object-Relational Mapping (ORM) libraries for Python. The library enables developers to perform tasks like defining models, creating queries, and managing database migrations, supporting multiple database management systems such as SQLite, MySQL, and PostgreSQL. By setting up a development environment, creating a Flask application, and using SQLAlchemy to create and manage databases, developers can build robust web applications with relational databases. The library also provides a flexible way to work with user data routes, allowing developers to add, update, and delete data in a database. Overall, Flask SQLAlchemy is a powerful tool for building web applications with relational databases, providing a flexible and robust way to work with databases in a Flask application.