Each post in most forums have a 'Thank You' button as a way of saying that you found that post helpful. Each member has a count of how many thanks given and and received. Using it is very simple - just click on Thank You and your name appears at the bottom of that post and everyone then knows you've said thank you to the poster.
You can remove the thanks you've made to other member's posts but you can't remove thanks given to your posts. Admin can at their discretion remove or lock thanks.
Please remember you must only say Thank You if the poster has provided some generally useful or interesting information. It must not be used to artificially inflate someone's thanks count or to support someone in an argument.
Obviously, you can't say thanks to your own posts!