This can be a tricky question and difficult to answer easily and concisely, due to the fact that flashing can literally be a game of inches, where flashing that is installed but is a couple of inches out of place or too short can lead to catastrophic failure.
Start with the concept that flashing is a material designed to “bridge” between dissimilar materials or conditions, to keep water out of the house. It is usually metal but can also be constructed of plastic or synthetic rubber-like materials. It needs to be integrated with the drainage plane within the building envelope to insure proper evacuation of water. Some parts of installed flashing will be hidden from view behind the cladding (siding), but some parts must be exposed to insure proper function. To shed water, individual components must be installed “shingle style” so that the upper pieces overlap the lower pieces. This approach will typically leave the lower edge of the upper pieces exposed to some degree.
Consider a roof configuration where a roof slopes down along a wall as on a two-story house. The intersection of the wall and the roof slope must be flashed to prevent water intrusion. The “L-shaped” flashing must be integrated with the drainage plane behind the cladding of the wall, and the wall cladding cannot extend all the way down to the shingles of the roof. The gap between the wall cladding and the roof shingles will result in exposure of the flashing material. At the lower end of the roof the flashing must be turned out from the wall (“kicked out”) to ensure water is directed out of the wall cavity. This piece of flashing would be visible and an indication of proper installation.
Alternatively, omission of such kick out flashing can result in devastating deterioration of wall framing within only a few years.
The configurations of flashing on homes can be extremely varied and complicated, and the opportunities for incorrect installation just as numerous.