A 25% tariff on Canadian goods going into America means: whoever buys those goods would be paying the tariff. If that's consumers, they will probably choose not to buy Canadian goods. If it's importers, they would have to pay it. Though, they might not buy the Canadian goods either, knowing people probably won't buy the imported goods at 25% markup. So, demand goes down. Meaning Canadians exporting won't be getting anything if no one is buying their goods.
It gives American companies the chance to stop importing that which could be manufactured in America and bring jobs back there, instead of importing goods with a 25% tax, thereby lower the cost of those goods. However, since Canada's only reason for existing as a country seems to be to have good a exporting/importing relationship, the tariffs are an opportunity to close our f**king border like it ought to be, and keep doing business with Americans. If we do that, Americans don't have to pay a tariff, and we don't have inventory gathering dust.