This resolution tells the President to pull U.S. military forces out of any combat involving Iran, unless Congress has specifically voted to declare war or authorize that military action. In other words, it's Congress asserting that only they, not the President alone, can commit troops to a fight with Iran. There's an exception built in: U.S. forces could still act in self-defense if the U.S. or American troops/facilities somewhere else in the world came under attack. So the practical effect is limiting the President's ability to start or continue military operations against Iran without Congress signing off first.