For anything to have been designed (such as a watch or computer or, as seen by Flew and others, the universe) it must show signs of having a designer. These characteristics must indicate the object is not self-caused - i.e., not a product of random, natural forces.
For example, when taking a hike through a rural hilly portion of northern Virginia, we would expect to see many rocks strewn about the woods - probably some randomly lying on top of each other. But we can make a distinction between a natural rock formation and a rock wall that may have been assembled during the Civil War. We do this by observing a couple conditions.