The idea is that China uses low wage workers and even slave labor to produce goods. When they get to ship them to this country without tariffs they can under cut basically any US company producing the same thing. The tariffs are leveling the playing field, allowing US companies to compete which, in turn, produces more jobs.
It works the same way cutting taxes helps the economy. Trump’s tax cuts gave companies tax cuts if they passed some/most of the savings to their workers and/or hired more workers. The initial reaction is that by cutting taxes, it will cause the government to have to spend more money they don’t have. But what actually happens is more income tax is generated and less money is having to be spent to support people that can’t get a job.
In both cases it is stimulating the economy.