The 2015 Nuclear deal was to allow Iran to generate nuclear energy at 1 facility for power and not weapons. In exchange for that the US and other countries could buy tens of billions of oil.
Trump exited the agreement as US was going towards energy independence and didn’t need their oil and Iran was building weapons vs just power. The US was no longer giving them billions of dollars to fund weapon development against their enemy, the US.
Biden’s administration is now discussing rejoining the agreement as they have shut down US energy production and are buying from overseas which has increased energy costs by approximately double to date in the first 3 months of Biden’s administration.

Additionally there was explosion at their facility yesterday. It is suspected that the US orchestrated it to get the oil cheaper from Iran and force their oil price down as the US needs cheaper oil since stopping a good portion of its own production 3 months ago.