Doze is meant to preserve your battery life. If your phone is at rest (not moving, charging, or utilizing your screen) applications are put in a hibernation state limiting their access to the network. Periodically applications are synced to get new alerts.
This feature is turned on by default, but can be modified for individual apps that you might want to monitor more closely. You can adjust the option by go to Settings > Battery, press the menu in the top right hand corner and select Battery optimization. Click on an app in question and select 'Don't optimize".