Apache JMeter is an open-source tool designed for performance testing and load testing of applications. It allows users to simulate multiple users accessing a web application simultaneously, helping to identify how the application behaves under stress. JMeter supports various protocols, including HTTP, FTP, and JDBC, making it versatile for testing different types of services. With a user-friendly interface, JMeter enables testers to create test plans easily and analyze results through detailed reports and graphs. This helps developers and quality assurance teams ensure their applications can handle expected traffic, ultimately improving user experience and system reliability.