Android apps on Chrome OS are not a new thing. In 2014, Google announced that it was working with a few select partners to bring certain apps to Chrome OS. Only a small number became available, and it was never really a consumer-facing project. Earlier this year, Google said that the experiment was scrapped in favor of a different system. Android apps would now run in containers, which would open the whole Play Store to Chrome OS users. This new approach would come to only some Chromebook models which had to be running the dev and beta channel builds.
Now, according to both the Chromium Projects page and the Chrome Releases blog, Android apps are coming to the stable channel for the Asus Chromebook Flip and the Acer Chromebook R11 / C738T. Google says that this update will be rolling out to these two Chromebooks over the next few days.
You might note that the Chromebook Pixel (2015) is missing. That is because it is still listed as only working with Android apps in the beta/dev channels. Google gives no explanation as to why, but it will hopefully be added soon.