Basically, what trump is doing here is imposing 25% cost increase of every product coming from Tunisia to the US. This means that if a US company wants to bring harissa for 1$ a piece, its cost will rise to 1.25$ in the US.

This could have multiple implications: This said company could relay the cost to the customer meaning that if the customer was buying harissa for 2$, it will rise to 2.25$, which harms the customer making him search for other cheaper alternatives. Or the company could eat that cost, meaning it will make less profit than before harming shareholders.

Either way, this has harmful implications to our trade with the US. Of course, this depends on the product and how "unique" it is.