A function prototype in C is a declaration of a function that specifies the functions name, parameters, and return type. It is used to provide function declaration and gives information to the compiler that the function may later be used in the program. A function prototype works like a function declaration where it is necessary where the function reference or call is present before the function definition but optional if the function definition is present before the function call in the program. A function prototype is not needed if the user-defined function is defined before the main() function. Function prototypes are often included in header files, which are then included in both the main program and the function definition files. They allow functions to be called from any part of the program without requiring access to the functions implementation details. By declaring function prototypes, developers can write more robust, maintainable, and error-free code.