You’re likely aware that these days many Chromebooks (certainly new ones) happily run Android apps, but it will reportedly soon be possible to sideload apps from outside the (relatively) safe haven of Google Play – at least for enterprise users to begin with.
At the moment, if you want to use Android apps from outside the Play store, it is possible to sideload APKs (Android Package Kits), but you need to be in developer mode to do so.