What's being described is the process of getting a law passed. It begins in the lower house (representatives), through the senate and finally to the office of the president. He has the right to refuse to sign the bill; he must provide reasons for so doing, or he can sign it, and it will become law.
The point is that no one part of government can control what becomes law without the consent of the two others. If ever there was a description of checks and balances, this is it.
