Der Brief wird geschrieben.
English: The letter is being written.
The passive voice (Passiv) in German is used when the focus is on the action or what happens to the object, not on who does it. In B1, you learn how to form the passive in the present (Präsens) and simple past (Präteritum).
Use the passive when you don't know who does the action, the person is not important, or you want to focus on the result or process.
Der Brief wird geschrieben.
English: The letter is being written.
Die Tür wird geöffnet.
English: The door is being opened.
Das Auto wurde repariert.
English: The car was repaired.
Die Hausaufgaben wurden gemacht.
English: The homework was done.