In 2015, world powers agreed to give Iran relief from some economic sanctions in return for inspections and limits on its nuclear program. Since the nuclear deal — formally known as the Joint Comprehensive Plan of Action — took effect in January 2016, Iran has allowed inspections and is seeing some economic payoff.
Meanwhile, President Trump was elected on claims the deal isn't tough enough. Those in favor of keeping the deal in place argue it's working and will prevent Iran from making nuclear weapons for 15 years. Those against it, like the president, argue it should last longer and also ban things like Iranian missile tests and support for militants.
Congress requires the president to re-evaluate — or certify — the deal every 90 days. The next deadline for him to do so is Oct. 15.