This function returns whether a given variable is a 64bit integer or not. In some cases you want to check and see what data type a variable holds in GameMaker Studio 2 and that's when you would use this function. It returns true or false depending on whether the value is an int64 or not. For further information on data types see here.
Argument Description n The argument to check.
show_debug_message("Not a 64bit integer!");
The above code checks the variable "val" to see if it contains an int64 and if it is not then it shows a message in the debug console.