You can give +K to your friends as a way of recognizing their influence. Perhaps you read a great article on marathon training that a friend posted; you can give your friend a +K about running as a way of recognizing them for providing great content and positively influencing you.
Just go to a user’s profile and click the +K button next to a topic to recognize them. You can give up to 10 +Ks each day. Your friends can give you +K in topics as well.