Typically when you look at your lock screen, you expect to see your wallpaper and perhaps some notifications. Some Android apps have been giving users an unexpected surprise in the form of lock screen ads, though, and so now Google is taking a stand against those apps.
Google has updated its developer policy with regard to apps and lock screen monetization. Specifically, Google now says that unless your app is specifically a lock screen app, you can't show ads or monetizing features on a device's lock screen.
Here's the official wording from Google's Developer Policy Center:
"Unless the exclusive purpose of the app is that of a lockscreen, apps may not introduce ads or features that monetize the locked display of a device."
This is a welcome move from Google. You might expect some lock screen ads from a lock screen app, especially if it's a free one, but it can be surpassing and annoying to see lock screen ads being display by an app like a photo editor or file viewer. Now that should no longer be an issue thanks to this new policy update.