Not really eli5 but: Iran is basically the USSR at the end of its days, hugely corrupt, inefficient, and still saddled with a broken planned economy that is collapsing into an oligarchy. It's very dependent on oil exports to stay afloat, with oil exports accounting for 80% of public revenue (in 2012, according to wikipedia), and while the sanctions began in late 2018 they weren't put into full effect until early May 2019. This is why you've seen a dramatic increase in tensions over the last month: because it just got very real for them.
Metaphorically, it's kind of like Iran was in a pool and wearing a snorkel and got dunked underwater in 2018, and it was fine enough because it could just breathe through the snorkel. But then in May the US put its thumb over the snorkel and now Iran is just mindlessly thrashing around fighting for air.
Man I'm going to traumatize my 5 year old.