Learn Hardcore Portuguese (Brazil): A reunião online foi remarcada para outra hora. - The online meeting was rescheduled for another time.

Portuguese usually uses a definite article (o/a/os/as) more often than English. With specific, known things like the meeting, you typically say a reunião. You can omit the article sometimes (especially in titles, headlines, or very generic statements), but in normal speech a reunião is the default for the meeting.

What’s the role of foi in foi remarcada?

Foi is the preterite (simple past) of ser: (ela) foi = (it) was. Here it helps form a passive construction: foi remarcada = was rescheduled.
So: A reunião ... foi remarcada = The meeting was rescheduled.

Why is it remarcada and not remarcado?

Because remarcada agrees in gender/number with a reunião.

a reunião is feminine singular → remarcada (feminine singular)
If it were o evento, you’d say foi remarcado.

Is this sentence passive voice? How would I say it actively?