An expert worker in Imperialism is a citizen who was (or, when he appeared as one of your capital's original citizens, was deemed to have been) a trained worker then put through the trade school with the submission of $1,000 and two vital pieces of paper to double his productivity in a single turn, making him the equivalent of four labor units. His evidence of qualification is the deep blue overalls.
The $1,000 may seem a lot compared with the $100 for the initial training, but even that is better value per labor unit than recruiting from the provinces (which costs one food can and one unit each of clothing and furniture - plus ongoing feeding - to get a single untrained worker), especially if you have a fair amount of money but not enough workers for your current production capacity and/or not enough food for a higher population.
Such workers can go into the army in a few important positions, such as Sapper.
They can, on the other hand, at any time be trained as civilian specialists. A civilian who is disbanded returns to the city as an expert worker.