For a while now, Google's Play Store has shown if an app contains in-app purchases, so you can better decide if you want to use it. Now, the store will also let you know if an app contains ads.
While free apps are extremely popular, developers still have to make money somehow. Sometimes that may mean supporting their apps with ads. Most of them don't say that up front, though. Now, Google's taking the guesswork out. If you're not interested in using an app that's supported by ads, you'll be able to find that out before you install it, rather than after you've already gone to the trouble.