The + sign?

It's to push the comment to the top of a thread :) They're labelled 'Top replies' and they appear in order of how many people upvoted them (clicked the + sign) 

I think people treat them as like or agree too though. I actually dislike having partial comments at the top, especially as they are only the start of a comment, and out of order and context, so I just scroll past them. I'd switch that feature off if i could! Although I do sometimes press the up arrow if I like a comment, but I'd rather it was just a normal like or agree type thing!

Yes, sorry, tried to imply this but wasn't clear. Usually I upvote comments I agree with/that express a point I agree with particularly well.

Yes, sorry, tried to imply this but wasn't clear. Usually I upvote comments I agree with/that express a point I agree with particularly well.