Backprop: no context around the 'step' notation

Hi,

Looking at the assignment exercise for backpropagation, I also went back to the lecture, and I’m seeing this:

However, I don’t think the ‘step’ function was ever explained. I’m not sure how to go about implementing the back prop function.

Thank you for any input.

Edit: My updated understanding is that the ‘step’ part is was is implemented in the assignment when going from variable z1 to l1.