I understand it’s an ambiguous title but I’d suggest the title is a little ambiguous until you know more. While it could stand for a writing implement called a ‘pen’ it has much broader connotations.
It began as “Poets, Essayists, Novelists” but has since been expanded to include “Poets, Playwrights, Editors, Essayists, Novelists”. The organisation began in London in 1921 and expanded very quickly. They support writers, and their families, who are persecuted for their writing. Sometimes that support will be writing to them in jail, sometimes campaigning on their behalf and sometimes offering financial support.
I am incredibly lucky living in a country where I have a guarantee of freedom of expression and will not be persecuted by the government for anything I write. Others are not so lucky and they continue to write anyway. By continuing their writing activities and highlighting inequalities within their society they risk persecution, jail or execution.
PEN International and its many member organisations support these people to the best of their abilities. Recently, during BookExpo 2017 PEN America was named the BookExpo Ambassador Award Winner. This is only the seventh such award, a previous winner was James Patterson.