Here's how that new button works:
If you like a post or have a positive comment to make to the poster, you can click on the thanks button. A comment box will pop up and you can enter text or opt to leave it blank.
The member receiving the thanks can view it via their user cp, just under the list of subscribed threads. The comments will be displayed as well as the date, name of person leaving the comment and a link to the post referenced in the comment.
Only the most recent comments are displayed. There are no prizes, swag or any other kind of forum favors that can be traded for thanks. It's just for fun and to help cut down on the number of one liner posts. This is another way to show approval.
If you don't want to receive the PM notice associated with a Thank You, you can disable that option in your user cp (edit options).
Wash your hands, flatten the curve - we can do this!