In C, functions are global by default. The “static” keyword before a function name makes it static. For example, below function fun() is static.

In C programming, a static function is a function that can only be accessed within the file it is defined in. It is a type of function that is not visible to other files that are part of the same project. This means that it cannot be called or used by any other file except the one in which it is defined.

Static functions are different from regular functions, which can be accessed from any other file that includes the header file that declares the function. One of the advantages of using static functions is that they can help to hide implementation details and reduce the risk of naming conflicts.

Static functions are declared using the "static" keyword before the function name. For example, the following code snippet shows how to declare a static function: 

static int myStaticFunction(int arg1, int arg2) {
    // implementation details
    return 0;
}

Note that static functions cannot be declared as "extern", which is used to declare functions that are accessible from other files. Also, static functions cannot be called from outside their file, even if they are declared in a header file.

In summary, static functions in C provide a way to encapsulate implementation details and reduce the risk of naming conflicts, by restricting their accessibility to only the file in which they are defined.