The 'Thanks' - System is a function that allows you to give back another user a positive feedback, for example he or she has posted a message you find useful or helps you to solve a problem.
How to give a User a Thanks?
If you want to give an User an 'Thank' press the 'Thank' Button inside the Post you want to add it, after that you find your Name in the List of User who gave a Thank at the end of the Message. You can also remove your Thank at any time.