Built-in annotations in Java are a powerful feature that allows developers to add metadata to their code. These annotations provide essential information to the compiler, enabling it to detect errors, suppress warnings, and facilitate various processing tasks. They can be utilized at compile-time, deployment-time, and runtime, making them versatile tools for enhancing code quality and functionality. Common built-in annotations include @Override, @Deprecated, and @SuppressWarnings, each serving specific purposes. Understanding and effectively using these annotations can significantly improve code readability and maintainability, while also streamlining the development process.