When a person feels you've done something good for another user, or for the feralheart game, they will give you a nuzzle, to show that they appreciate it, or want to encourage you to do more. Its just for fun! c: Once you hit 25 posts, you can nuzzle others.
Like Decessei Stated above, It used to be named "karma" though, before people could "bite" you to show if you've done something wrong.. This time, no one can bite, only nuzzle, so that no one gets their feelings hurt and such. Also, Floof-o-meter seems a bit more relaxful and fun!