The Phone app that is found on most stock Android smartphones (due to the fact that most other manufacturers use their own app for the dialer), is updated regularly by Google in the Play Store, and v13 just came out a few days ago. Some users are already getting the update on their devices and noticing a new feature, which brings video calling straight to the dialer app. This is something that other apps have had for a little bit, and Verizon has it in its own dialer app on its smartphones. But Google has now brought Duo video calling straight to the Dialer, making it easy to get that video call started.
This brings some pretty small changes to the Phone app on your Android smartphone. But some of the changes you'll see is the new option to start a video call while you are on a call. There is also now a video call icon next to contacts that Duo installed on their smartphone (remember it's available for both Android and iOS), and finally, there is also a small card at the top of the screen when viewing recents, telling you about the Duo integration with the Phone app on your smartphone. So there aren't many big changes with this functionality, and it's not even necessary for everyone to use it. However if you do, you'll want to download Duo - which if you have a stock Android device, it's already pre-installed.
Remember, that while video calling is a nice feature to have, it is going to cost you quite a bit of data. As you'd expect, video calling use quite a bit more data than a simple voice call over LTE or WiFi using VoLTE or VoIP technology. So if you don't have an unlimited plan, you'll want to be careful with using this feature. Additionally, if you or the person you are calling, is in an area with somewhat spotty internet bandwidth, the stream will look like its in SD or lower resolution instead of HD, and that's simply due to the bandwidth that is available. If you don't already have the latest version of the Phone app, you can get it from the Google Play Store.