Hi Switchplus,
What you can do is to get the value selected from the dropdown, then query the dataset for that value and then assign the result.items[0].description to the text element.
$w("#descriptionText1").text = result.items[0].description;
For more information on how to Query, please check this - https://www.wix.com/code/reference/wix-data.html#query
Hope this helps!
Best,
Mustafa