It is simple PHP language style of declaring and calling a function! We are essentially calling a function which was defined/declared in functions.php or any included file.
In PHP we can call any function with its name and (); e.g. for a function named myFunction, we can call it as myFunction();
http://www.w3schools.com/php/php_functions.asp
This is same as we call other built-in WordPress functions like, get_the_title(); or the_title(); etc
And to declare your own custom functions in functions.php
You may use this syntax:
function your_function_name(){
//your php code here
}
Then as functions.php is included by WordPress in all theme files, you can then simply call this function in any theme php file as:
your_function_name();