Free trade means that each nation removes any tariffs on items being brought in from another country. The argument against free trade is that you want to protect a domestic industry. So we might put a $2000 tariff on cars from Japan because they cost $2000 less than American cars.
Free trade allows nations to compete more equally. Even so, there are issues. China, for example, is accused of subsidizing its industry. This makes their goods artificially cheaper, so they sell more in the US. The US industry might ask for tariffs on Chinese items to make the competition fair, or to bias the competition towards the US.
The problem is that consumers are hurt when there are tariffs. Our stuff is more expensive, and so we make less because we sell less, and the economy does not grow. Free trade grows the economy much more quickly than placing tariffs in an effort to protect US jobs. But tell that to the folks who will lose their jobs. They don’t care if they can buy stuff cheaper. They don’t have a job.
But eventually they will get one. And be better off. That’s why free trade wins out over tariffs.