Good morning, unchecking the install from outside sources will not stop your app from working. Leaving it unchecked will not harm your device, in fact it will help you know when you are installing apops or files from outside the Google eco system. It is recommended that you try to refrain installing apps from outside the Google store (unless your absolutely sure who did the app) because you stand a better chance of picking up malware, viruses and other issues that can actually take control of your device and hold it for ransom until you pay what they are asking. It is always best to download apps from the Google app store this insures that your device stays safe and running free of viruses and other potentially dangerous items. Hope this helps. (Foot note; I'm not saying don't download apps from outside sources, there are many admirable apps, but they're are also many dubious apps as well, just make sure you know exactly what you are downloading. If your not sure it's best not to do it.