Android 6.0 has added new power-saving modes, Doze and App Standby, prolonged battery life by up to 2 times according to Google's measurements. The Doze mode has been created to improve the sleep efficiency of idle devices, while the App Standby mode has been designed to prevent apps from eating up power while in the idle state. On both occasions, plugging in the device to chargers allows normal operations to resume.
Dozing is when a device is unplugged, the screen is off, and it's stationary (this can be determined via sensors, such as the accelerometer) for a determined period of time. What we get is a state where the system is kept in the sleep state as long as possible. When an Android 6.0 device is in the Doze mode, not much will happen in the background, as shown in the following figure:
In short, everything you think will happen in the background will not actually happen.