Original: Jet streams, or just jets in context, are fast flowing, narrow air currents found in the atmosphere of planets at the tropopause, the transition between the troposphere (where temperature decreases with height) and the stratosphere (where temperature increases with height).
Simplified: Jet streams are fast flowing, relatively narrow air currents found at the tropopause, the transition between the troposphere (where temperature decreases with height) and the stratosphere (where temperature increases with height), United States Department of Energy.