Answer: What this means is that an author will state the purpose of their writing to the readers in the text or if they don't then its up to us the readers to reread the text and figure out what the purpose of their writing is ourselves.
Step-by-step explanation:
When an author states their purposes explicitly they normally do so by a thesis statement or in a straightforward way.
When they don't outwardly tell the audience what the purpose of their writing is at the beginning or end the readers have to go back and pick up contextual clues throughout the writing in order to figure out what the authors purpose is. This could include a variety of things such as; tone of voice, descriptive imagery, location, values, ideology, etc.
I don't know if this will help all too well, but I hope it assists in you getting closer to the right answer!