IDN stands for International Domain Name. This name has an extended letter and character range. This can also include umlauts such as German Ä, Ö and Ü as well as diacritical marks from other languages such as French, Chinese, Indian, Swedish, Arabic, etc.
In addition to the 26 letters and the numbers and certain special characters from the ASCII character set, the IDN contains 92 additional characters.
In German, umlaut domains are typical examples of IDN.