This function returns whether a given variable is a string number or not. In some cases you want to check and see if a variable in GameMaker Studio 2 holds a string and not a real and that's when you would use this function. It does not return the string but rather true or false, so a value of, for example, "fish" for n will return true, but a value of 200 for n will return false.
Argument Description n The argument to check.
name = "Player: " + val;
The above code checks the variable "val" to see if it contains a string and if it does it adds it into another string which is then assigned to the variable "name".