Reference

If, on the other hand, your code uses the static keyword, the x variable gets allocated to memory the first time the function runs. And the x variable just stays allocated in memory for the duration of the entire program. You've just saved yourself some reads and writes to RAM:

some_function() {
    static int x = 5;
}

Global Variables versus Static Variables

Static variables are actually placed in the same area of RAM as global variables. The difference is that global variables are declared outside of functions and are available anywhere in your program to any file or function. On the other hand, static variables remain in scope. So in the above example, some_function() is the only place that can access the x variable.