Fair use lets people use copyrighted material without asking in some situations, such as criticism, news reporting, teaching or research. US courts weigh four factors: the purpose of the use, the nature of the work, how much was used and the effect on the market for the original.
It is at the centre of US lawsuits over AI training. Because it is decided case by case, early rulings do not settle the question for every model. Fair use is a US idea; the EU and UK rely on specific listed exceptions instead.