The Unknown sources option should be enabled whenever you need to Install the APK file. In other words, If you want to Install the Apps which are not available in the Play Store, then, this option should be turned on.
1. Select the Download Button in the html page to download the APK
2. A dialog titled "Download file?" with a warning message will appear on your phone letting you know you must give permission to download the apk file. Select the download button.
3. Tap the Settings button in the dialog box to proceed.
4. On the next page, you will be asked to allow the app to make APK installs. Toggle on the Allow from this source
5. A prompt should pop up, giving you the option to Install the app. Click the button allow installation from this source.
6. Note - Sometimes you may need to initiate the download again.