What is a de minimis threshold?
If you're shipping an order to a foreign country, the de minimis threshold is the value at which the destination country will start assessing duties and taxes. If the retail value of the parcel is under the de minimis threshold, then the customer won't have to pay any duties and taxes; the country has decided for whatever reason that it is not worth the effort. If the retail value of the parcel is over the de minimis threshold, then the customer may have to pay duties and taxes to their country's government.
For example, at the time of this writing, the Canada tax de minimis threshold is about USD 15.00. If you ship a USD 10.00 phone case to someone in Canada, then they won't get dinged with taxes. If you ship two phone cases totaling USD 20.00, then the customer will have to pay tax on that. The tax is usually collected by the carrier and is usually equivalent to the sales taxes or VAT that they would have paid had they bought the item in their own country.
When importing goods into the United States, the de minimis is currently USD 800.00, so it has to be a pretty big shipment for it to take effect.