Basic guidelines of "no using the old UI widgets (grey keyboard, old tabs, old back arrows)" and, old by now, supporting iPhone 4 and iPhone 5+ screen sizes with optimized graphics. Same goes for iPad except there's only one resolution to support even though it comes in two resolutions across two sizes.
"iOS 7 Optimized" is Apple's way of saying "built with the iOS 7 SDK".
Apps that are built for iOS 7 get the new shiny keyboard and some different behaviour when run on iOS 7, but can still run fine on older versions of iOS if the app developer is willing to support older versions of iOS. This is done by setting an option called "deployment target" to an older iOS version when building in Xcode (and testing a ton on older versions of iOS, and not calling newer APIs on versions of iOS that don't support them).
So this announcement means Apple is dropping support for older versions of Xcode (with older iOS SDKs), but older versions of iOS are still technically supported as long as you're building with the iOS 7 SDK.