Credit rating agency Fitch puts U.S. AAA rating on watch. Here's what that means.

Credit bureau Fitch Ratings said the AAA rating of the U.S. — the highest available — is at risk because of the "brinkmanship over the debt ceiling," which is increasing the risk that the nation could default on some of its debt and other obligations. 

Late Wednesday, Fitch said it is moving the U.S. to "rating watch negative" due to the increasing risks that the nation will default, although it added that it "still expects a resolution to the debt limit before the x-date." The so-called x-date is the day when the Treasury will run out of money to pay its bills, and which Treasury Secretary Janet Yellen has estimated will arrive on June 1.