This is the best summary I could come up with:
Fitch, one of three major independent agencies that assess creditworthiness, cut the rating from the top level of AAA to a notch lower at AA+.
When Congress returns from its summer recess, lawmakers will have to work to reach an agreement on next year's budget before the end of September to prevent a government shutdown.
"Treasury securities remain the world's preeminent safe and liquid asset, and... the American economy is fundamentally strong," she said in a statement.
However, Nobel Prize-winning economist Paul Krugman said "the biggest economic news over the past year has been America's remarkable success at getting inflation down without a recession".
The move "should have little direct impact on financial markets as it is unlikely there are major holders of Treasury securities who would be forced to sell based on the ratings change," he added.
I'm a bot and I'm open source!