A software library is a collection of pre-written code that developers can use to perform common tasks without having to write the code from scratch. These libraries provide functions, classes, and methods that simplify programming by allowing developers to focus on building their applications rather than dealing with low-level details. Examples of software libraries include NumPy for numerical computations and React for building user interfaces. Software libraries can be specific to a programming language, such as JavaScript or Python, and can be open-source or proprietary. They help improve productivity and code quality by promoting code reuse and standardization. By using libraries, developers can save time and reduce errors, making software development more efficient.