A GPU, or graphics processing unit, is a chip designed to do a huge number of simple calculations at the same time. A CPU has a few powerful cores that tackle tasks one after another; a GPU has many more, smaller cores that work side by side. It was built to draw video-game graphics, where every pixel can be worked out at once.
That same parallel design suits AI, which is mostly multiplying large grids of numbers. Most AI models today are trained on GPUs, and many data-centre GPUs are now built almost entirely for AI rather than graphics. You meet the term in almost every story about AI computing power, data centres and chip export rules.