Previously, the only option available in the app was to "stream at highest quality only", and otherwise the app would automatically stream the highest possible quality depending on your mobile connection. The new update adds new options so you can better control your mobile data usage, and aside from these settings the update promises to use less data overall. Probably connected to that, the update also makes it faster to save music to your device for offline use.
Other than that, the only change is "search quality improvements". As we said, the update is rolling out now, so you should see it soon enough. And, if you haven't tried out Google Play Music All Access yet, you can still get one month free if you sign up before June 30th. Google Play Music is available for free in the Google Play Store.