xn-- on domain, what it means?

This is Punycode which is used to Internationalize Domain Names in Applications.

From 1:

Punycode is intended for the encoding of labels in the Internationalized Domain Names in Applications (IDNA) framework, such that these domain names may be represented in the ASCII character set allowed in the Domain Name System of the Internet. The encoding syntax is defined in IETF document RFC 3492.

From 2:

Internationalizing Domain Names in Applications (IDNA) is a mechanism defined in 2003 for handling internationalized domain names containing non-ASCII characters. These names either are Latin letters with diacritics (ñ, é) or are written in languages or scripts which do not use the Latin alphabet: Arabic, Hangul, Hiragana and Kanji for instance. Although the Domain Name System supports non-ASCII characters, applications such as e-mail and web browsers restrict the characters which can be used as domain names for purposes such as a hostname.

Its the result of IDNA encoding; i.e. converting your unicode domain name to its ASCII equivalent which has to be done as DNS is not unicode-aware.

The xn-- says "everything that follows is encoded-unicode".