A past participle is just a verb conjugation that describes something that is over and done: written, said, cooked, closed etc.
A porta está aberta. (The door is opened.)
O projeto é completado. (The project is completed.)
Ela está cansada. (She's tired.)
Ele foi morto. (He was killed.)
*The endings must take the gender of the subject:
a porta está aberta
o escritório está aberto