It's a a 'Like' button (as per FaceBook and Twitter) - for posts only, not comments.
However, it appears that it only works on the new style - which means that I for one won't be able to use it (even if I had intended to).
I assume, therefore, it means that if someone who uses the new style and thus sees this little button uses it on a post of mine that I won't be able to see the 'Like'. And, of course, it will apply to anyone else who doesn't use the new style. Having said that, apparently paid account users can view their feedback statistics, so maybe those of use who do not use the new style will be able to see if anyone has 'Liked' a post we make - which seems a somewhat long-winded way of doing things, but there you go.
For more information and/or to comment as to whether you like this feature or not go here.