You are trying to install or open an app that didn't come from the Mac App Store but you got a message saying App can't be opened because it is from an unidentified developer. This is how you bypass it.
If you are afraid your friends, kids or any other person you are borrowing your iPad to, might delete an app by mistake or even intentionally, you can enable restrictions on individual all of the apps.
If for some reason your Android powered smartphone does not perform as good as it used to, or you want to sell it I suggest you to bring it to its original factory state.
Tasty drinks like our grandmothers used to do it, they were tasty, easy to make and full of vitamins, let's make a smoothie out of oranges, bananas and apples.
The circular grass creations, that look like something out of a different planet scattered throughout the Namib Desert have long been one of the mysteries of science.