It hibernates or you can say disable the power draining apps and settings when the phone is in sleep mode, i.e. screen is off; so that when you wake up your phone again to use, you will have more battery power to use and the apps and the settings which were disabled will be enabled automatically. You don't have to toggle on or off the settings each and every time.
Hibernate app also can lock CPU to the lowest possible frequency ensuring huge battery saving and enhanced performance.
It only works when the screen is off, without interfering your day to day mobile usage. You can choose the apps and settings, which needs to be hibernated and practicaly can forget about the hibernate app, it will keep working in the backgroud to ensure battery saving and enhanced performance of your device.