The String class is built-in to Flash. Just as there are many operations we can do with numbers, such as adding and subtracting, there are many operations we can do with Strings.
var s:String="ABCDEFGHIJKLMNOPQRSTUVWXYZ";
trace(s.
When you type s. the list of methods will appear.

Type trace(s.charAt(3));
You will see the letter D in the output window. Like most things in computers, numbering starts with 0. Thus, s.charAt(0) is the letter A.