InDesign has always had the ability to export a file that can be opened in a version of InDesign one version back. In the past, this was achieved by choosing File > Export and choosing the "InDesign Interchange (INX)" format. The resulting INX file can then be opened by the next-oldest version of InDesign.
Starting with InDesign CS5, the INX format is gone, replaced by the "InDesign Markup (IDML) format. This format functions exactly like the previous INX format for opening in the previous version of InDesign.
The IDML format is also useful for resurrecting a damaged InDesign file, as I wrote about previously.
Why the change? The IDML format is a well-documented XML-based format that makes it possible for developers and scripters to automate InDesign document creation and modification, beyond what was possible with the old INX format.