In one sense, the answer to this question is very simple. In another sense, it is complicated and is far beyond the scope of an answer on this site.
Put simply, the North American Free Trade Agreement (NAFTA) created a free trade zone consisting of the three countries, the United States, Mexico, and Canada, that make up North America. A free trade zone is a zone in which there will be no barriers to trade between nations. Many nations set up trade barriers between themselves and other countries. For example, they might impose tariffs, which are taxes on imports that are coming into their country. As another example, they might impose quotas that limit the amount of some particular import that can enter their country. Economists argue that trade barriers are very harmful. They argue that countries will, on the whole, benefit when trade barriers are lowered. Because many government officials believe in this idea, NAFTA was approved by the governments of all three North American countries. In this view, then, NAFTA decided to tear down trade barriers between the US, Mexico, and Canada.
Looked at differently, the answer to this question would be huge. NAFTA is a tremendously detailed agreement that consists of more than 1,000 pages of terms. What this means is that the NAFTA negotiators had to decide on a myriad of issues. They had to decide on exactly how trade barriers would be taken down and by when this would have to happen. They had to decide if any exceptions would be made. They had to decide on how to enforce the treaty. In other words, whenever you have an agreement that runs over 1,000 pages, you have many things being decided. From this perspective, we can say that huge numbers of little details were decided by the NAFTA agreement.