Trade deficits are created when we purchase more foreign goods than we sell to them. Tariffs are a major contributing factor. If we have tariffs at 5% (simplifying this), but the foreign country is charging us 100%, then we will be importing more than we export to that country.
What Trump has down is to raise tariffs to send the message that this must stop, with the goal of eliminating them altogether, or as much as possible.
Free trade means that politically-connected corporations are unable to rent-seek with respect to foreign commerce. Things like NAFTA and TPP aren't free trade, they are essentially world leaders negotiating which industries will be free to slap tariffs. Leader from country A, for example, might not be able to impose tariffs on a certain industry because that industry has a very powerful lobby he is beholden too, but other, less politically powerful industries, are fair game. So what you end up with is "okay, we'll keep these industries free if you do the same for those industries"…effectively its world leaders deciding who wins and who loses in the market.
It stifles commerce, hurts innovation, domestic manufacturing, etc…its government deciding who wins and loses instead of the market. And, of course, it artificially increases prices for imported goods, which, in turn, harms exports….Manipulated trade is a disaster for the economy.