Mass production meant a growing share of the work in factories was done by

Machines

Explanation:

In business, mass production refers to the practice of increasing the output of production (finished goods) as much as possible.

In order to do this, a business need to eliminate as much inefficiencies as possible within the production process. Most of those inefficiencies often come from the limitation that we have as humans. (our body need to rests, so we couldn't continue the production after a couple of hours).

Machines do not possess such limitation. We can make the machines to continue the production as long as we have enough power to turn it on. So, growing the share of machines to do the work is crucial for mass production.