I couldn't understand the second issue

Hi. I'm having some trouble to understand some points of the video:
- The phrase "Differentiate the values from zero". What do you mean exactly? In what context/phase of the model should I apply it?
- Also, I can't hear clarely at 1:28 - 1:31s, and don't know what the instructor says.
Could someone help me? Sorry for my bad english.
Regards