Normally when you close applications in iOS, the operating system permits those applications to run in the background for up to 10 minutes before they're suspended in a "deep sleep" mode.
A new free jailbreak tweak called SmartClose
by iOS developer Ryan Petrich will make it so that applications in iOS are automatically sent into that "deep sleep" mode when you close them, rather than being allowed to stay open in the background.
Petrich explains how this works on Reddit:
Originally Posted by Ryan Petrich