Update (7/14/2015): An agreement was reached between a group of six nations (including the U.S. and Iran) to scale back the Iranian nuclear program in exchange for sanctions relief. All parties involved made concessions of one form or another, and while there has been vocal opposition to the deal — Congress will have the opportunity to express its disapproval and override a threatened presidential veto to stop it.
This bill was originally about changing how fire departments offer healthcare to their volunteers (click here for that bill's summary). After being passed in the House in it's intended form, it was repurposed in the Senate as the legislative vehicle for offering Congress the power to approve or reject a Nuclear deal with Iran. The Morning Call explains:
"The U.S. Senate is in the midst of debating a proposal to give Congress the right to review any nuclear agreement with Iran. To get the proposal through Congress quickly, senators are planning to use Barletta’s bill, replacing his idea with their Iran review proposal ... [Sen. Pat Toomey (R-PA) is] offering an amendment that, instead of stripping out the volunteer firefighter provisions, would keep them in the bill when the Iran proposal is added."
In its new form, the bill would require the President — within five calendar days of reaching an agreement with Iran relating to their nuclear program — to submit the text of the agreement and all related materials to relevant congressional committees.
The Foreign Relations/Affairs Committees from both the House and the Senate would hold hearings and briefings as necessary to review the agreement during the 60-day period following the President’s delivery.
Throughout this 60-day review period, the President would be prohibited from waiving, reducing, or limiting in any way the current sanctions on Iran. Actions specified in the agreement could only be taken if Congress adopts a resolution favoring the agreement. The deal could not go through if Congress develops a resolution to oppose it.
If Congress fails to pass a joint resolution either favoring or opposing the agreement, only actions authorized by existing law would be permitted.