The jet stream is a relatively narrow band of strong winds near the top of the troposphere, typically around 30,000 feet, that separates air masses of different temperature and steers weather systems from west to east. NOAA JetStream describes it as a river of air whose speed peaks at the core and falls off on either side.
Where the jet comes from
The jet forms where the horizontal temperature contrast between air masses is strongest, most sharply along the boundary between polar and mid-latitude air. That temperature gradient tilts pressure surfaces sharply with height, and by the thermal wind relationship the wind aloft accelerates through the tilted layer.
The result is two main features in each hemisphere. The polar jet sits between about 50 and 60 degrees latitude and drives most mid-latitude weather. The subtropical jet sits near 30 degrees and is generally weaker but plays a larger role in winter.
How forecasters use it
Jet position and orientation together answer three practical questions. Where will surface cyclones develop and travel, because they form under and downstream of the jet. Where will the strongest deep-layer shear sit, because the jet contributes most of the upper wind. And where will the corridor of lift set up, through the jet streaks and their divergence patterns.
In the cool season, a jet dipping south over the Plains often supports classic severe weather days. In the warm season, the jet retreats north into Canada and the severe weather corridor drifts north with it, though slower southerly upper flow can still support storms.
Where the picture is oversimplified
A single line drawn as the jet flattens what is really a broad, layered flow with embedded jet streaks and vertical wind maxima at multiple levels. Two forecasts that both show a similar jet position can produce very different storm days depending on where the streaks are and how the surface pattern is oriented under them.
The jet also does not create instability on its own. A strong jet over dry, capped air can pass without producing storms. The jet is the top of the setup, and it has to align with a moist, unstable surface layer for the day to deliver.