Back in May, Trump's administration said they'd reimpose sanctions on Iran by the 6 August. The time has come, and neither Iran, nor US allies, are very pleased about it.
What it means: Sanctions are sometimes described as 'economic warfare' - essentially refusing to engage with another country on economic terms if you're not in agreement with their politics.
In 2015, an international agreement lifted sanctions that'd been on Iran for years in exchange for Iran agreeing not to produce so many nuclear weapons.
Trump never liked this deal, and in May, he said he wanted out. He's re-applying sanctions in two stages: this stage involves banning trade of gold, steel, coal, Iranian currency, and US dollars. The next one involves energy, petroleum, and anything coming in and out of an Iranian port.
Other countries aren't following suit, but Trump's said if his allies don't join him in re-imposing some of these sanctions by November, he may retaliate on them too - with as yet unspecified 'financial measures'.