Activity Monitor is a built-in application on macOS that allows users to view and manage the performance of their computer. It provides real-time information about system resources, including CPU usage, memory usage, disk activity, and network activity. This helps users identify which applications or processes are consuming the most resources. Users can sort and filter the displayed information to focus on specific tasks or applications. Activity Monitor also allows users to force quit unresponsive applications, making it a useful tool for maintaining system performance and troubleshooting issues on their Mac.