Alongside his promise of 25 million jobs, Trump has also said he can get America growing at 4% a year again. That would be almost double what Obama achieved. The U.S. hasn't seen consistent 4% economic growth since the tech boom of the late 1990s.
Of course they matter. Trade deficits arise when we import more than we export. Easy said. It's a tad bit more complicated. When we keep incurring these deficits, year after year, we use our domestic currency (the $) to pay for those imports. This results in our trade partners getting a considerable supply of those $.
Exports directly increase and imports directly reduce a nation's balance of trade (i.e. net exports). A trade surplus is a positive net balance of trade, and a trade deficit is a negative net balance of trade.