Most buildings and areas that are placed require workers to produce resources. You have to manage how many workers are in each profession, and modify where they work as your town grows, faces disasters, and changes.
Citizens that have already been assigned a profession but have completed all available work begin to do work fit for a general laborer. To maximize the use of your citizens, make sure there is always skilled work, or change them to a different profession.
The profession window shows how many citizens are working in each profession, allows you to assign them, and shows how many jobs are currently available.
Increasing the number of workers in one profession will reduce the number of general laborers by one. Likewise, decreasing the number of workers from a skilled profession will increase the number of general laborers.
Pressing the focus button will focus and cycle through all citizens working in the profession.