Finding tablet apps on the Play Store has historically been a bit of a trial-and-error process. It usually involves finding an app and hunting through the screenshots for a tablet picture. If there isn't one, you install the app anyway and hope a tablet layout will magically show up.
With Android tablets gaining more and more market share, Google has decided to turn up the heat on app developers that don't build apps with tablets in mind. Apps that don't include certain tablet enhancements will display a "designed for phones" tag on the tablet version of the Play Store, making users with tablets less likely to install them. Google is also upping the ante on the front page and "Top app" lists on the tablet Play Store, which will now hide phone apps by default.
Google has a tablet quality checklist for developers hoping to avoid a public shaming. There's also a tablet compatibility section in the "optimization tips" section of the Android Developer console, which will scan an app and offer tailored recommendations for improvement.
Still, seeing an app without the "designed for phones" tag does not necessarily mean that it will have a tablet-optimized layout. Two of the biggest Android Tablet deniers, Facebook and Twitter, won't get tagged with Google's new scarlet letter.
While Android tablets used to be a distant second to the iPad, most recent market share reports have Android tablets outselling iOS tablets. Tablet app design hasn't really kept up with market share, so Google is presumably hoping that a gentle prodding will get developers to upgrade their apps to take advantage of larger screens.