Doze Mode is a set of changes and rules that put your phone to sleep when idle.
On Android 6.0 Marshmallow: Doze mode gets activated after a while the screen is off, the device is stationary and it's running on battery. As you can see in the diagram above, when Doze Mode gets activated, the device doesn't get any wakelocks, network access, jobs/syncs, Alarms, GPS/Wi-fi scans.
On Android 7.0 Nougat: Imagine if your phone is on your pocket (the screen is off, it's running on battery, but it's not stationary) you might want to get the Doze Mode features as well, right? So that's why Google announced the Extended Doze Mode: It runs when the screen is off, but not stationary. As you can see in this diagram, only Network Access and jobs/syncs are disabled. Note that the Extended Doze doesn't replace the first Doze Mode. They work together, depending on the phone state (stationary or not). Here are the distinctions: Developers should be aware that:
You can find more information here: https://developer.android.com/training/monitoring-device-state/doze-standby.html