Germany PhD application: "Work Certificate"
German here.

I guess that's just a questionable translation of the standard German phrase Arbeitszeugnis (pl.: "-se"), Arbeit=work and Zeugnis=certificate, diploma.

In business, Arbeitszeugnisse would mean a collection of diplomae, plus (relevant) testimonials from your former employers, or qualification certificates.

In your case, it'll probably a small collection, maybe just the diploma. That's absolutely fine at this point of your career. But if you already happen to have some experience as a lab/teaching assistant, or have worked in a related industry branch during non-term, providing some evidence might be a bonus.