A file descriptor is a unique identifier used by an operating system to manage open files and other input/output resources, such as sockets and pipes. It is typically represented as a non-negative integer, allowing programs to read from or write to these resources efficiently. When a file is opened, the operating system assigns a file descriptor to it, which the program can then use to perform various operations. In many programming environments, such as those using the Unix or Linux operating systems, file descriptors are crucial for handling multiple files simultaneously. The standard file descriptors include stdin (0), stdout (1), and stderr (2), which represent standard input, output, and error streams, respectively.