These are fast-flowing air currents high in the atmosphere. They occur on Earth, as well as on planets such as Jupiter. On Earth, there are major jet streams in the Northern and Southern Hemispheres. Planes can sometimes use the push offered by jet streams going in their direction to cut travel times by riding these winds over long distances.
In a sentence
Warmer winters can make the Northern Hemisphere jet stream “wiggle,” which can change the weather further south.