Out of the box, your Android device only allows you to install apps from Google Play Store. To install software anywhere else, you have to enable installation from third-party sources. That is, enable the “Unknown Sources” setting.

1. From the Homescreen go to Settings >> More.
2. Go to Security >> Device administration.
3. Check "Unknown Sources"

This will enable you to install apps from sources other than the Play Store.

You may need to adjust this according to your Android models. For some users, simply go to Settings -> Security and make sure Unknown Sources is checked. Sometimes you meet the problem of can’t downloading apps.