Nova Launcher is one of the most popular third-party launchers on Android, partially because it's always adding innovative new features. That is certainly the case with v5.1 beta 1, which adds something called "dynamic icons." Rather than a boring numeric badge, these are badges with the content from notifications in them.
To hear developer Kevin Barry tell it, badges with numbers aren't very useful. What's the difference if you have 5 messages or 10? And then there are some apps that don't even count notifications the same way (eg. messages vs. conversations). Dynamic Badges instead seek to provide useful information about what's happening in the app.
The new beta grabs images from your notifications, attaching them in badges to app icons. So, a new text message results in a badge with the contact's profile picture. That's more useful than simply seeing a "1" there. Playing music? The album art can be shown as a badge.