Trump has quit a deal between the US, its European allies, and Iran and said he's going to hit Iran with the toughest economic sanctions he can.
What it means:In 2015, the US, along with its main European allies, including the UK and France, plus Russia and China, signed a deal with Iran. The deal promised to lift economic sanctions – a form of economic penalty, placed on one country by an other, if it disapproves of its behaviour. They can be things like trade barriers or tariffs, which can hurt a country's industry – if Iran curbed its nuclear development program.
According to the Guardian, the deal led to a rapid reduction in Iran’s nuclear programme. It reduced its stockpile of low-enriched uranium by 98% to just 300lbs, not enough to make one bomb. But Trump's said the deal's failed, that it hasn't led to peace and that he is going to come down hard on Iran.
Although the other European countries involved are pretty desperate to keep the deal (politicians from France, Germany and the UK have all been in the US in the past few weeks to try and persuade Trump to stick to it) they're now going to have to fight pretty hard to keep it standing. One analyst said that with economic sanctions imposed from the US, global companies could leave Iran, further damaging its industry.
There are lots of different theories about what could happen next. Some people think the US siding with Israel, and Saudi Arabia, over Iran could lead to war in the Middle East. Others think that Iran will now start making weapons again.