"The result, then, is that more plentiful and better-quality goods are more easily produced if each person does one thing for which he is naturally suited, does it at the right time, and is released from having to do any of the others."
By this quote, Plato means that in a just city, each person does only the job they are naturally best at, and this way, every job gets done as well as possible and each person only has to worry about that job, not meddling in any other business. This principle of "specialization" will keep the city protected from contamination of jobs done improperly, and it will keep the city running smoothly.
Specialization is a very important theme in the book. The just structure of the ideal city, according to Plato, depends and is based upon the principle of specialization, that each member of the city plays a specific and important role according to his nature, and does not meddle in any other business. He believed that only this strategy could ensure that each and every job was done properly and as well as possible. It keeps everyone doing the job they do best, and no job is done by someone who is any less than the best at tackling it.
Plato believed that this was the way to conduct the ideal form of political justice. Everyone must do what best suites their nature and not pry into any business other than that which is naturally suited to them. According to Plato, his society was separated into three roles: Producers, Rulers and Guardians. Producers produce the goods (carpenters build, artists paint, doctors heal, etc), philosophers rule, and Guardians fight and protect. Specialization guarantees that each class remains in fixed relations of power. Warriors abide by the commands of the rulers, producers to not interfere with political affairs, each person worries only about their own business to which they are suited, participating in other aspects only as far as obeying the rules that are created by the rulers,