Slavery was a legal institution in Ancient Greece, as was citizenship. Aristotle's account was grounded in the common social and political structures of his period.
In most Greek city-states, people were divided into three categories: citizens, metics (resident aliens), and slaves. Citizens had a variety of rights, such as the ability to own property and bring lawsuits. Slaves were legally classified as property, just like cattle, chickens, or even pottery. A slave had no rights and was not considered a person under the law. Even women who were not slaves were limited in their rights and freedoms, being under the control of their fathers and then their husbands. Metics or resident aliens were neither slaves nor citizens but simply foreigners, who had no legal rights or citizenship. Some cities, such as Athens, had very restrictive criteria for citizenship, leading to something of an underclass of metics. The term "master" simply refers to a person who owns a slave.
Aristotle justified this system with a concept of "natural slavery," arguing that some people were not sufficiently rational to make wise decisions and thus were best suited to lives as slaves.