What is an APK?

An APK is the install file of an Android app. A store usually opens and installs that file for you. If you save and open the file yourself, the phone may ask for an extra permission.

An APK from a page you do not know may not be the real copy of the app. When you can, install from a known store and check the developer name you see. The file name alone does not show that it is safe.