The essence of a trade deficit is that exports are less than imports from the point of view of a country with a trade deficit. This is traditionally seen as undesirable because in effect the market for that country's goods is smaller than its trading partner and therefore lower demand is lower return
You can examine this by examining the country of "You". As a grown human you regularly trade your only resource (your labor) to an employer for the employer's resource (cash). You take your paycheck and buy everything that enables your to produce more labor. In that case there is no trade deficit since "cash" can be converted to anything you want.
A labor trade deficit can illustrated by the use of the company store. Many large employers would use the company store and company housing as a form of causing a trade deficit with the labor. That is instead of trading you cash for labor, they would give you credit at the company store, housing and a little cash. This had the effect of trading something that had a value disparity between the two parties. For example if housing costs $500 to a normal laborer that gets a paycheck it actually costs less to the provider. So the company provides $500 of value to the employee which cost the company less. Additionally the laborer cannot covert that $500 of value to anything else (such as living in a smaller home, or even building your own home and saving the rest).
To tie this to a country's economy it can be argued that if a country can produce a resource for less cost than another, that country can sell that resource for less than other countries. The other countries have no choice but to buy that resource and not produce it themselves.
Here is where there are two schools of thought on why this is not desirable.
Trade Deficits don't matter
If a country can buy enough of a resource from another country doesn't that already mean that that country is rich enough to afford it?
Trade Deficits are bad
The costs of a resource is now double for me since I have to buy the resource and I miss out on the opportunity cost of not producing it myself.
These two statements are oversimplifications of course, there are more social issues associated with not producing a resource such as large unemployment of the blue collar class, large disparity between the classes and military strategic issues (If there is a war, I can't produce a resource since there is no one with the knowledge of how to produce that resource)
On the other hand, the trade deficit country, because of technology, can dismiss all these issues, for example, blue collar workers can be re-educated to other trades, everyone benefits from lower prices, and the world is smaller because of technology so if there is a war I can buy whatever resource I want from any place in the globe because I have huge cargo jets.
My opinion is that human nature cannot be change by technology, people vote with their gut, so unemployment makes people feel bad and showing that what they chose as a career is obsolete by retraining also causes bad feelings, technology can fail, often in a catastrophic cascading event.
Business men like Trump know that markets also run on feelings. Only by destabilizing the status quo can there be a chance of changing those feelings. It is a gamble with no assured winners. It goes against good politics. However, remember "Too Big To Fail", that worked for him before. What I mean is that we are China's biggest customer, will that give us enough leverage?