APKs are the files for the apps on Android. But it's very dangerous to just download and install APKs from not very serious sites on the internet.
Google Play is always the most safe way to install apps, but sometimes you may want to install a newer version which isn't already on Google Play or maybe it's not available in your country. For this case there's APKMirror.
APKMirror is a site build and maintained by the Android Police team, one of the most popular Android related news sites. On APKMirror everyone can upload a new APK file, but the team will check every APK before they publish them.
So if you want to get the latest version for example of Google+ for Android just head over to APKMirror and search for it.
But: before you can install the downloaded APK, you first have to enable the option â€œInstall from unknown sourcesâ€ inside the security settings on your phone!
I hope I was able to help you exploring new ways of how to be up to date ?.