It's the ability to be in the here and the now, and to not focus on past events or future possibles, things which cause stress, anxiety, and often depression.
I don't think you should be emotionless. I think that, in a way, you shouldn't be anything, you should just be there in that moment. By that, I mean, you should just let what ever emotion that comes up be, and just watch it and understand it. Same as thoughts, you shouldn't be fighting your thoughts during meditation, you should just be watching...