Question

What are jet stream
Function of jet stream

Answer 1

Jet streams are the super high speed winds blowing in the region of upper atmosphere.

Before understanding how it exists or works you need to understand these three cells (not the ones in biology), as shown in this image ( source:https://pin.it/ylaoetzj4n4eda).

So we know that ground or water near equator get heated up and runs into the atmosphere after that it cools and moves to the places of low pressure around equator.
And this process is repeated contineously making a cycle.

And this completes what we call Hadley's cell in northern and southern hemispheres.

This same thing happens on tropic of cancer and Capricorn making another cell called Ferrel's cell.
amd Similarly, polar cells are formed.

And between the boundaries these two cells thjs jet stream exists.

Depending upon their location tropical amd subtropical jet streams are classified in groups.

In India what we see is tropical jet stream.

Now,
when ITCZ ( Intertropical convergence zone) shifts, (that means that the region of Earth receiving the most heat is shifted upwards or downwards depending upon the revolution of Earth) this subtropical jet streams (not only subtropical but tropical too) shifts a litter upward or downward respectively.

Answer 2

These are narrow belt of high altitude westerly winds in the troposphere.