Article one of the U.S Constitution aims to establish everything regarding the legislative power of the nation and particularly, regulating the organisms and the functions that are given and are a part of this legislative power. First, it establishes that the control of the legislative power falls on the U.S Congress and also the state legislatures. It also establishes that the Congress will be divided into two houses, the House of Representatives and the Senate and sets the functions and powers given to each house. It establishes how the representatives to each of the houses will be selected and the periods of power they will have. It also establishes the division of power between the three main branches of government: executive, legislative and judicial.
In this particular excerpt, the article is refering specifically to the constitution of the legislative power, who will be able to be a member of this branch of government and the functions and limitations as well as the reaches that this branch will have.