Once you reach 50 posts you will have access to a +1 button that you can use to applaud a helpful or quality post. The reason it takes that many posts to reach is because the system (and it's inverse, the -1 smite) were being misused and abused by a lot of new people to the forum. In the meantime, before you get there, there is absolutely nothing wrong with replying "thank you" to people you feel have been helpful. 🙂