The Zone 2 feature allows control of either the same, or separate, source than the one being listened to in the main room, in another location. For example, the user can be watching a Blu-ray Disc or DVD movie with surround sound in the main room, while someone else can listen to a CD Player in another room, at the same time. Both the Blu-ray Disc or DVD player and CD player are connected to the same Receiver, but are accessed and controlled separately using the same main Receiver.
The Zone 2 feature is usually limited to analog audio sources. However, as you move to higher-end home theater receivers, you might find, in some cases, that the provided Zone 2 option might accommodate analog video digital audio sources as well. In fact, a small number of midrange and higher-end receivers also provide HDMI audio and video output for Zone 2 access.
Also, some higher-end receivers may include not only a Zone 2, but also a Zone 3, and in rare cases, a Zone 4 option.
When shopping for a home theater receiver with a Zone 2 feature, check what specific signal sources are included.
For more details on how Zone 2 (and additional Zones) are implemented on home theater receivers, refer to my supplementary article: Home Theater Receivers and the Multi-Zone Feature as well as additional article How to Use the Multiroom Audio Features in a Stereo or Home Theater Receiver (from About.com Stereos).
Also, here are some related articles on home theater receivers: