From the experiment set up in the diagram, the pointer is resting on the drinking straw while the rod is resting on the drinking straw.
When the rod is being heated through the bursen burner, there will be linear expansion in the rod. As the rod increases its length, this causes the drinking straw to roll and thereby causing the pointer to rotate.
The pointer therefore rotates because of the thermer expansion that happen in the rod due to the heat from the bursen burner.
