What does ROR mean and why does it have to be on time for roasting?

ROR = Zrate of rise (RoR), which means what degree per minute seed temperature rises. It's derivative to the temperature curve, not the temperature itself. If the temperature rises fast, ROR is high. If it goes slow, ROR is low. If it stabilizes, ROR approaches zero.

why is it important: the declining ROR steadily from the beginning to the end of roasting is generally producing coffee with more developed and balanced flavor. The fluffy ROR or the one that suddenly rises again at the end is often a more "harsh" or inconstant one.

concrete number: start Zroasting after charge ROR can be 15-25 degrees per minute. Ideally declining continues, entering first crack at 7-10 degrees per minute, and approaching end of Zroast at 2-5 degrees per minute. It's a public basis, not a golden rule.