IDN stands for "internationalized domain name". Historically domain names could not contain accented characters or characters other than Western European language characters. IDNs liberalize that.

Here's a non-authoritative source for further reading: http://en.wikipedia.org/wiki/Internationalized_domain_name

Thank you!

Boy, I wish people wouldn't assume everyone is as knowledgeable as they are when they post update notes. It's pretty frustrating for someone like me, who doesn't know all the jargon.

You're right. I think in North America hardly anyone has heard of this issue. Or if they have, it's how they need to be careful about fake websites where someone substituted a nearly-identical-looking Cyrillic character in the domain name.