Extendable software frameworks are pre-built structures that provide a foundation for developing applications. They include reusable code, libraries, and tools that help developers create software more efficiently. By using these frameworks, developers can focus on building unique features rather than starting from scratch. These frameworks are designed to be flexible, allowing developers to add new functionalities or modify existing ones as needed. This adaptability is crucial for meeting changing requirements and integrating with other systems. Popular examples of extendable software frameworks include Django for web development and Spring for Java applications.