The JavaServer Pages Standard Tag Library (JSTL) is a collection of useful JSP tags and functions for web application development in Java. It provides a set of standardized tags to simplify and streamline the creation of web pages, allowing developers to focus on the business logic rather than the presentation layer. JSTL includes tags for controlling flow, accessing data, manipulating XML documents, and handling internationalization, among other tasks. It is a valuable tool for Java developers who want to create dynamic, data-driven web applications.