How to Save iPad Power by Closing Apps
Pressing the Sleep/Wake button on top of the device turns the screen off, but it doesn’t stop apps from running.
If you intend to pick up where you left off with an app, pressing this button will preserve whatever you were last doing but lock the screen. However, running apps drain battery faster, even with the screen off. If you’re done with an app, click the Home button to close it before turning off the screen.