Installing apps from outside of the Play Store can be a good thing (I mean, it's the whole basis for APKMirror), but it can also lead to trouble. For some time now, a user would have to go into the Security settings and toggle "Install from unknown sources." One of the many changes in Android O is to how this model works — that old option is gone and each app now must be granted the install source permission for APK installation.

So this is based on where the downloaded file comes from. For instance, if you get a file from APKMirror, Chrome needs the install source permission before you can do anything. And anything that's trying to do something nefarious and trick you will be blocked. Here are some screenshots to show what I mean:

We are in agreement here at AP that this is probably a good way to go and a nice step forward for user safety. Basically, you will be able to identify what is asking for permission to install something. I think this is for the best and it is not that much more inconvenient than the old way.

Thanks: Zsolt