Displays the percentage of likes to views, more popular videos have a higher ratio.
Youtube recently removed the dislike to like ratio.
A video with 1M views and 100 likes for example has a low ratio (0.1%), and probably means the video is not popular.
Any video with a like to view ratio greater than 2 or 3 percent, should be all right.
However the correct ratio depends on the type of content you are watching.