A stub is the beginning of an article (in an encyclopedia) or a subroutine (in a computer program).
It can provide a simple definition or serve as a placeholder.
Some writers use stubs as seeds, hoping that a great article will blossom from what is planted. Sometimes the first words of an article can set the tone or establish a direction or scope which provides lasting value to the article. In one case at Wikipedia, the opening phrases remained essentially unchanged even after 2 1/2 years.