Refer to figure. 
Jet streams are narrow bands of very strong winds located near the tropopause, typically between FL250 and FL400. They form due to strong horizontal temperature gradients, especially along fronts and air mass boundaries. 

Decoding the wind speed involves interpreting the following key indicators: a short line represents 5 knots, long lines indicate 10 knots each, and a "flag" corresponds to 50 knots. These values are cumulative when determining the overall wind speed. 

As you can see in the attached figure, the highest wind speed in the jet stream is 170 kt (3 flags ; 50 x 3 + 2 long lines ; 20 x 2).