Hi Lolo
The 'Thank You' link is for forum users who want to thank other forum users for things like helpful advice.
Say for instance that there was a part or an accessory that you wanted to buy for your caravan, but were not sure where to buy it from.
If you asked on the forum and someone helped by adding a url link to a store or accessory shop which stocked the item that you were looking for they would have helped you, so you could click on the 'Thank You' link underneath their forum post.
The thanks that they received would appear in their Avatar profile box.