The Android system doesn't need you to close a running app for every instance. But in some cases you may need to force stop an app that's misbehaving, or like playing a game, you may need to actually close the app in order to start a new level rather than continuing the last. Here are the steps to follow:
Run the Settings app and tap Apps under DEVICE.
Select the app you want to close.
Tap the 'Force stop' button and tap 'OK' to confirm