NAFTA stands for the North American Free Trade Agreement. This agreement was controversial for the same reason that most free trade agreements are controversial. It was controversial largely because many people believed that it would, as H. Ross Perot famously said, lead to a “giant sucking sound” of jobs leaving the US for Mexico.
When rich countries make free trade agreements with poor countries, people in the rich countries inevitably worry about the loss of jobs. With free trade, the people in the poorer countries (in this case, Mexico) will be able to compete directly with those in the rich country (the US, for our purposes). People in Mexico tend to be willing to work for much lower wages than do people in the US, largely because those in Mexico are poorer and have lower costs of living. This led many Americans to fear that American jobs would be lost as factories moved to Mexico to take advantage of lower costs there.
Similarly, there were worries about things like environmental standards and labor rights. The basic fear was that Mexican firms would be able to outcompete US firms because they did not have to adhere to strict regulations in these areas.
Thus, NAFTA was controversial largely because many people believed that it would hurt the US economically by allowing American companies to be outcompeted by lower-cost Mexican firms.