HDR is a photography mode that stitches together several pictures along a range of exposure settings. Using various algorithms, the effect creates pictures that can have fewer dark or washed out spots than a conventional digital still, which is great if you intend to capture textures and detail instead of glare or shadow.
Apple introduced HDR photography to iOS 4.1, but the feature wasn't rolled out to every device. The devices that currently lack the option to enable HDR in the stock camera app include the iPod touch 4G, iPad mini, and the iPad 2 to the iPad 4. Lucky for us, this disabled feature is fairly easy to manually reintroduce on a jailbroken device...