This will be kept simple. To explore Aristotle in depth probably exceeds both my competence and the interest of my handful of readers.
According to Aristotle, before one examines the purpose of a political community, one must understand the purposes of life for individual members of that community. In Nichomachean Ethics, Aristotle attempts to answer that question—what are the purposes, ends, or goals of human life. In a follow up work called Politics, he explores the question of the best political arrangements for community life.
It is in the first book, however, that he introduces political science and the object of its inquiry.
"Every art and every kind of inquiry, and likewise every act and purpose, seem to aim at some good: and so it has been well said that the good is that at which everything aims."
Because there are many kinds of arts (skills) and inquiries (sciences), there are many ends or purposes. Two examples Aristotle puts forth are managing economic matters and waging war. To Aristotle however, these various pursuits are limited activities with limited ends. They exist as means to or constituent parts of a larger, supreme, or highest end or purpose. Aristotle wants to know what is this supreme end or highest purpose. To discover this, Aristotle says we must know what, in his words, is the "master-art" or "master-science" to discover that supreme end or highest good.
Aristotle asserts that this master science is political science--the science of the state.
"Since then it [politics] makes use of the other practical sciences, and since it further ordains what men are to do and from what to refrain, its end must include the ends of the other sciences, and must be the proper good of man."
The object of politics, then, is to discover and bring about the proper end, purpose, or good for mankind. Aristotle is less concerned with mankind as individuals than as member of a community.
"For though this good is the same for the individual and the state, yet the good of the state seems a grander and more perfect thing both to attain and to secure; and glad as one would be to do this service a a single individual , to do it for a people and for a number of states is nobler and more divine."
Before one can understand the what is good for a community, one must apprehend what is good for its individual members.
And thus Aristotle begins his exploration of ethics and politics