The perception is that while we allow free access to our markets, other countries - the same ones that ship boatloads of cr@p to our market from overseas - have barriers to our exports. Years ago, it was Jap cars exported to the USA, but their market was closed to many US exports, most notably rice. After years of increasing friction, Japan, Inc., addressed this by building factories and such in the USA, so you can now buy a Jap car designed in California, engineered in Michigan, and built in Kentucky, mostly by US workers and mostly with US-sourced parts. We still have a lot of Jap cars on the roads, but they're not actually imports for the most part.
We have similar issues today with other countries, especially (but not exclusively) China; as I understand it, Trump wants their markets to be as open to US products as ours are to theirs. (China manipulates its currency too, which is a related issue.)