Newbie question about String.charCodeAt(); method

I just have a basic question ( I am learning the String Class). I don’t get the return value of this method. Example ( from Flash Help) :
var my_str:String = “Chris”;
var firstChar_num:Number = my_str.charCodeAt(0);
trace(firstChar_num); // output: 67
How can it be 67 ??? I tried several values but I dont get it.
Thank you.