The Spring framework is often referred to as a "lightweight" framework because it is designed to be lightweight and non-invasive. This means that it does not require a lot of code and does not heavily rely on specific design patterns or architectures. Instead, it is designed to be flexible and adaptable, making it easy to use in a variety of different contexts and with a variety of different technologies.
One of the key features of the Spring framework is its use of inversion of control (IoC) and dependency injection (DI). This allows developers to create modular, reusable code that is easy to test and maintain. The Spring framework also provides a number of useful features and tools for developing web applications, including support for web sockets, REST APIs, and security. All of these features contribute to the framework's reputation as a lightweight, easy-to-use tool for building modern applications.