The jet stream consists of ribbons of very strong winds which move weather systems around the globe. Jet streams are found 9-16 km above the surface of the Earth, just below the tropopause, and can reach speeds of 200 mph.
The position of a jet stream varies within the natural fluctuations of the environment. They are caused by the temperature difference between tropical air masses and polar air masses. What happens in one part of the world depends on what is happening elsewhere - the atmosphere is a complete environment with numerous connections.
Waves or ripples along the jet stream can cause Atlantic depressions to deepen explosively as they are steered towards the UK, so they are very important to meteorologists.
Met Office forecasters work in one of only two centres in the world that produce weather charts for global aviation, detailing the location, height and strength of forecast jet streams and the turbulence associated with them. Trans-Atlantic aircraft take advantage of this information - this is why flights home from the USA are often much quicker than outbound flights.