From the Super Mario Wiki
A Flower Grapple is an object in Super Mario Galaxy 2. These grapples appear usually in levels where Mario finds Yoshi, as the dinosaur is able to use them. Yoshi can use his tongue to take a grapple and then swing from flower to flower to reach higher areas. To do so, the player must point at a grapple with the cursor from the Wii Remote and then press the button, so Yoshi will stick out his tongue towards the object and swing from it.