Here, with an excerpt on the plain English:
Plain English summary
Every contract is a deal between two parties, and both are supposed to perform their part of the deal. That means that, if the job covered by the contract is done as it should be, then that party is entitled to be paid what has been promised. This case involved Indian tribes that sued the government because it did not pay all of the costs it had promised to cover when the two sides made their deal for the tribe to provide education and other government-like functions for their members. The Court ruled that a promise is a promise, even if the government doesn’t have immediately available enough money to pay all of the contractors it had promised to pay for their services. Congress has to locate the money to cover such a promise, the Court said.