Look at the alt key code. You hold the alt key then hit the numerical sequence, then release the alt key and voila! the item is written. e.g. hold alt, type 129, then release the alt key and you will get: ü : a letter u with an umlaut.
Umlauts are used primarily in German, from which we get the term, and are used to alter the pronunciation of certain letters.
Although I forgot to point out that most alt key codes are not supported by AG. If you use them here you'll likely end up with some rather strange looking characters
