If you are trying to take a list of lat/lon values and add in an attribute for county name (or other descriptive information), you have a few choices:
1. do it outside of Tableau as a data processing step. If you have a geographic file with county boundaries (e.g., the US Census has US County boundary files), you can add your lat/lon points and the Census data to a GIS like QGIS and do a spatial join.
2. do it in Tableau (starting in 2018.2) by converting your table with lat/lon values into a geographic file (e.g., shapefile) and using the spatial intersection functionality that is added in 2018.2 to join the table with lat/lon to a spatial file with county boundaries.
Thanks for quick reply. I want to do through tableau and not sure how to convert lat/long file to geographic file & also how to use the spatial intersection? Any demo would be much appreciated.
The spatial intersection join is a feature that will be released soon in Tableau 2018.2, so it is not currently available (but will be soon!)
In terms of converting a lat/lon file to a spatial file, I find the easiest way is to just add the file to QGIS and save as a shapefile.
Importing CSV files to QGIS: https://www.qgistutorials.com/en/docs/importing_spreadsheets_csv.html
Saving as Shapefile: With the new layer in QGIS, you should be able to just right click and Save As... and save it as a shapefile.