You’ve undoubtedly happened upon an app on your iPhone in which tabs appear at the bottom of the interface for navigating said app. Many native apps, in addition to third-party downloads from the App Store, use this type of navigation interface.
Users generally tap on the tabs at the bottom of the app to get from one view to another, but a newly-released and free jailbreak tweak called FluidTabs by iOS developer tomt000 brings this concept to the 21st century by allowing users to ‘swipe’ between an open app’s tabs.