You need workers to help you in your factories (Dairy, Pottery, etc.). Without them, you will not be able to craft any items. The more factories you have, the more workers you will need.
In order to know the exact number of workers you have, tap on any of the houses for workers. There you'll see the total number of them and the number of workers that are free and available at the moment.
In order to know whether you have enough workers available just tap on any factory you want to use. In each factory, the second number shows how many workers the factory needs to start operating. The first number will either be the same as the second - meaning you have enough workers and can start the production, or will show you how many workers are available if you don't have enough - in this case the first number will be lower than the second.