Tariffs on aluminium and steel imported into the US from the EU, Mexico and Canada were supposed to come in today.
What it means: The planned tariffs are all part of Trump's plan to stimulate US industry by making it more expensive to import stuff from abroad. He's already put tariffs on products from China, which has put the countries on the brink of an all-out trade war.
The EU has said it'd retaliate to any tariffs by doing the same on 'iconic' US products like Harley-Davidson motorbikes, and bourbon whiskey.
The decision to postpone the introduction of tariffs came after the Wall Street stock market suffered yesterday – the stock market acts as an indicator of how confident people are feeling in US business. If things are going great, they invest lots of money, and the market becomes more valuable. If they're worried about the economy, the opposite happens, as happened yesterday.