What is this line on my dock?
Solution 1:
This is a progress bar under Downloads folder. It sometimes doesn't go away after completing the downloads. One solution for this is to enter this command in Terminal:
killall Dock
This will reset the Dock.
Solution 2:
Moving the downloads folder to a new position on the Dock and then moving it back takes care of the problem without using Terminal. Great solution for non-technical people! :)