What exactly is the argument for Free Trade? There's been a lot of talk about Free Trade. People seem to either know nothing about it, or they seem to either support or oppose it with such passion that if you didn't know any better, you would think they were actually at war.

Now, I often don't like so called "Free Trade Agreements" like NAFTA, TPP, RCEP, CETA, etc. but more often than not my aversion to them tends to be based on toxic copyright provisions and a lack of Fair Use protections, not necessarily the actual macroeconomic effects of the trade deal as a whole.

Thus, I have to ask: what exactly is the argument in favor of Free Trade? What exactly is it trying to achieve? And are there ways a nation can address the negative effects of such deals? And what is Free Trade NOT?
The theory behind free trade is denominated comparative advantage, and as mike stated, the advantage is that a country, as a whole, benefits from specializing in the fields at which it is best, as it will get more than it would if it would produce everything alone.

In practice, a big advantage is the ability to operate for a bigger market, reaping economies of scale. This is particularly advantageous for small countries.

The downside is that, although free trade will make the country as a whole richer, it wont make the society necessarily equally richer, so it wont guarantee that an economy will develop. This connects with the concept of exploitative trade and fair trade: by entering international trade, developing countries might leave their economies vulnerable to foreign companies or governments, sometimes in critical sectors such as food and energy.