The function fopen is used in programming, particularly in the C and PHP languages, to open a file. It allows developers to read from or write to files on the system. The function takes two main arguments: the name of the file and the mode in which to open it, such as read, write, or append. When a file is successfully opened using fopen, it returns a file pointer that can be used for further operations like reading or writing data. If the file cannot be opened, fopen returns a false value, indicating an error. Proper error handling is essential when using this function.