Joel Mokyr writes in Demand vs. Supply in the Industrial Revolution that

The determination of 'when,' 'where,' and 'how fast' are to be sought first and foremost in supply, not demand-related process.

I am confused as to what is meant by "supply" and "demand" in this case. What are specific examples of supply-side explanations? Would it be similar to the argument that supply creates its own demand due to overproduction leading to excess inventory and price drops? Any help on understanding the terminology would be great.