NAFTA is the acronym that stands for North American Free Trade Agreement. It is an agreement that was signed in 1993 by President Clinton. It basically orders that there should be free trade between Mexico, the United States and Canada. Free trade means that there should not be tariffs or other trade barriers on goods being traded between these countries.
Economists believe that this is a good thing. They say that free trade is always good. They point to the fact that trade between the three countries has gone way up and that the countries are at least as well off now as they were in 1993.
Opponents say that the low wages in Mexico take jobs away from Americans. They say that industrial farming in the US hurts Mexican farmers. So they say that the agreement helps big companies but hurts ordinary people.