AWS Lambda External Extensions are a feature that allows developers to add code to their Lambda handler functions in separate processes, improving maintainability and performance. This separation of concerns enables developers to implement features like logging without modifying the original Lambda function code, reducing maintenance work and increasing resilience. By running external extensions in parallel with the main Lambda function, these extensions can leverage multiple CPUs, leading to improved performance and reduced impact on user experience. Additionally, external extensions can execute code after the main function returns, allowing for additional processing without affecting the user. Overall, AWS Lambda External Extensions are a powerful tool for developers to improve the scalability, reliability, and maintainability of their Lambda functions.