Flashing refers to weatherproofing a window. The material itself, which can be metal – aluminum, copper, zinc, or steel – vinyl, or tape, is called flashing and is installed around the window to fill any gaps and prevent water from entering the building. Incorrectly installed window flashing, or a lack of flashing, can allow water into the wall cavity and lead to wood rot or insect infestation.
However, the act of installing it is also referred to as flashing. Videos and articles are available online on installing and flashing windows correctly.