Google's equivalent of Apple's minimum-functionality guideline: an app has to be stable, complete, and provide real functionality — not a placeholder shell, and not one that crashes on common devices.
Most often caused by testing only on a high-end device and not against Android's much wider hardware/OS fragmentation, so the submitted build crashes on common lower-end configurations.
"Your app crashed during our review, or does not function as described in your app's store listing, per the Functionality policy."