Why is this important? Well, from Google and its users’ point of view (which probably means you and your web site’s visitors) they don’t want to show more than one result in a search with essentially the same content. It would be very frustrating if several of the results links took you to basically the same page.
Duplicate content is considered "substantive blocks" of very similar content on two or more web pages. This doesn’t, however, apply to the same page translated into more than one language (so no need to worry if you’ve got a French version, a Spanish version, etc.).
The post gives some good pointers on how to ensure that the version of the page you want gets indexed (e.g., you may have a print version that you don’t need to have indexed, but the regular version you do want).