epoll is a scalable I/O event notification system used in Linux. It allows applications to efficiently monitor multiple file descriptors to see if they are ready for reading or writing. This is particularly useful for servers that handle many simultaneous connections, as it reduces the overhead associated with managing these connections. Unlike traditional methods like select or poll, epoll can handle a large number of file descriptors without significant performance degradation. It uses a kernel-level data structure to keep track of events, making it faster and more efficient for high-performance applications, such as web servers and network services.