It’s a tax increase which can be (and is being) mis-portrayed as something that the seller pays, when in fact it’s the buyer that pays it.

In practice what Trump did was institute the equivalent of an additional 25% sales tax for all Americans when they buy goods manufactured in Canada or Mexico, but because this tax is usually payed by companies (which do most of the importing) and most people aren’t at all familiar with how Import/Export works, he seems to be getting away with portraying it as a tax on Canada and Mexico.

(The concern of those countries is not that they pay more - which they don’t - it’s that a selective “sales tax” that only applies to products they export to the US makes their products less competitive on price when sold in the US, hence they will sell less which is bad for their companies)

I’ve seen some theories around that the purpose of this significant increase in tax is to pay for the tax cuts for the wealthy that the Republicans are passing.