The acronym IDN stands for 'Internationalized Domain Name'. The IDN system allows internet users to use the full alphabet of their language in their domain names. They're no longer restricted to the English A-Z, and can use the full Latin character set, as well as characters from other languages, including Chinese and Japanese.
IDNs work by encoding the domain name using a system called Punycode. When you wish to register an IDN domain, you must convert the domain name to Punycode, and register the encoded version of the domain name. Then when the user enters a URL containing an IDN domain into their web browser, it will convert the IDN domain into Punycode and resolve that domain.
Not all TLDs support all languages. You will need to select the language that matches the characters you are using when you check out and purchase the domain name.