Java libraries are collections of pre-written code that developers can use to perform common tasks without having to write everything from scratch. These libraries provide reusable functions and classes, making it easier to build applications efficiently. They cover a wide range of functionalities, from data manipulation to graphical user interfaces. Some popular Java libraries include Apache Commons for utility functions, JUnit for testing, and JavaFX for building rich desktop applications. By leveraging these libraries, developers can save time, reduce errors, and focus on the unique aspects of their projects rather than reinventing the wheel.