The Constitution divides power evenly between three branches. This is to provide a system of "checks and balances." If any one branch tries to overstep its authority, the other two have the responsibility and power to keep it in check.
Article 1 establishes the Legislative branch and
Section 8 thereof grants it very specific authority.
The Legislative Branch writes bills that the Executive Branch either signs into law, or vetoes.
It is divided into two "houses" (The House of Representatives, and The Senate) which collectively are known as, "The Congress."
In The House of Representatives, each state is represented according to its populace. This ensures that the more populous states receive more representation to conduct "The People's" business.
In the Senate, every state, regardless of size, is represented by two senators. This ensures that the smallest state receives the same representation as the larger states, in conducting "The State's" business.
Bills may originate in either house, (however, "All Bills for raising Revenue shall originate in the House of Representatives...") and if they pass, they are passed on to the other house. If a bill passes both houses, before it becomes law, it is presented to the president for his signature. If he does not agree with it, he may "veto" it (Veto is Latin for, "I forbid.") and send it back to the congress with his written objections. If two thirds majority in both houses overrule the president's veto, the bill becomes law as if the president had signed it. (
Article 1 Section 7)