This setting enables or disable the ability to install apps that come from unknown sources, such as sources other than Google's Play Store.
Most devices are pre-configured to trust apps that are from the Google Play Store, and allow these to be installed.
This setting controls whether or not you can install apps from sources other than the Google Play Store. Enabling this is essential if you wish to install apps from bare .APK files you download from the Internet. It also may be necessary if you wish to use an alternative, third-party app store.
Please note that this enabling setting has important security implications. Apps that are installed from untrusted sources may be harmful to your device or your privacy. When this setting is enabled, misbehaving apps or websites may find it easier to trick you into installing untrusted apps.