A jet stream is a narrow band of strong winds several kilometres above ground. The flow of air in a jet stream can be likened to a fast-flowing river. Where the fast-flowing river meets slow-moving water near the banks, waves and swirls are often generated. When these waves and swirls get big enough, the water there may become rather turbulent. The same applies to a jet stream. This explains the bumpiness experienced by aircraft when encountering CAT near a jet stream.
Below is a weather chart, prepared by the Hong Kong Observatory (HKO) for flights departing Hong Kong, showing winds and temperatures at an altitude of 30,000 ft (around 9 km). The red line marks an area with winds exceeding 80 knots (around 150 kilometres per hour), revealing where the jet stream was at that level. The pilot is alerted to possible CAT through the significant weather chart.