Mark as Solution

Most forums have an option called "Mark as Solution".

This is a button that shows up for the post author on the bottom of the posts when there is at least one reply.

Clicking this button will then let other users know you found this response valuable and that it answered your question.  

It will do several other functions to let people know the post has been answered.

It adds a link to the top of the post.

It adds a checkmark next to the post on the thread listing letting others know it has been solved.

You also add to that poster's reputation, letting others know they have solved one or more posts.  That little bubble is also clickable, leading you to a listing of their solved posts.