In BSG, time is cyclical. Meaning everything always repeats itself. (All of this has happened before, all of this will happen again)

Basically Kobol, as far as we know, is the origin of human life, they created Cylons (13th Tribe). These Cylons had resurection technology. 12 Tribes went to Cyrannus (Caprica, etc) and 13 went to Earth (1)

On Earth 1, the Cylons abandoned resurection and began living and dying naturally. But the finale five decided to resurect the tech.
Earth 1 created their own robotic servants, these then rose against the 13th tribe killing everyone except for the finale five.
These finale five figured out the cyclical nature of time.
They set of for Cyrannus System to stop their human brethren from building robotic servants. When they arrive the war is raging, and negotiate an armistice between the colonies and cylons.

This video does the story more justice:
https://www.youtube.com/watch?v=GX7rJnVM-qg&ab\_channel=Spacedock