When did this start? After Trump announced a large increase in US tariffs last Wednesday.
And a tariff is... A tax paid to the government to bring goods into the country. Most countries have some tariffs. The US's were relatively low, but Trump has increased them significantly.
Why did he do this? Trump's been saying he wants to do this for decades. The US buys more physical goods than it sells, and he claims this is unfair and threatens security. Tariffs, he says, will bring more jobs to the US.
But most experts disagree... Most economists argue trade barriers such as tariffs reduce growth, and that such a quick and dramatic shift in tariff policy will make trade and investment harder.
And how do other countries feel? Not great. Many have threatened to retaliate by increasing tariffs on US goods entering their countries.
This affects stock markets because... Companies are facing higher costs to move their products across the world. There's also great uncertainty - it's not clear what Trump, or the countries he's putting tariffs on, will do next, making investment decisions more challenging.
And stocks are what exactly? To own a stock is to own a share of a company - the more you have, the more of the company you own. A stock market is where people buy and sell these shares. So when a stock market index falls, it means the value of those companies have dropped.
Some more response to Trump's tariffs, this time from their northern neighbour.
The Canadian government has requested dispute consultations with the US at the World Trade Organization (WTO) over Trump's decision to impose a 25% tariff on cars and car parts from Canada, the international trade body says.
According to the WTO, Canada argues the tariffs are inconsistent with the obligations the US has under several provisions of the 1994 General Agreement on Tariffs and Trade (Gatt).
