1:1
On many past occasions means the time before the Messiah's arrival. For the author of Hebrews, this mainly meant the Old Testament era. During that time, God spoke often and in various ways through:
dreams,
visions,
powerful acts,
stories,
commands,
encouragements,
angelic appearances, and
appearances of God himself.
The prophets were all those people through whom God shared his message (see 2 Peter 1:19–21).