I am very curious to know about what happens when the Stack Overflow users votes up on comments and what does it means "1" near by comments ? I have also voted up on few comments , having this question for long time, please let me know if you are aware about this.
When a user finds your comment meaningful, they simply votes up it. It shows one kind of indication, how useful your comment is.
You won't get any points for it, however there is a badge called pundit for Left 10 comments with score of 5 or more
Comments that are upvoted by community users are considered more important than comments that aren't voted up quite so high or that aren't voted up at all.
You generally see the results of this more on MSO or on popular questions on Stack Overflow when there are lots of comments on a single post. When the comment thread gets long enough to where it collapses, the comments are ordered so that the higher voted comments are displayed, and lower voted comments are hidden in the collapsed view.
When you click "add / show X more comments", all the comments are displayed in their original, chronological order.