From the Super Mario Wiki
A Flower Grapple is an object in Super Mario Galaxy 2. These grapples usually appear in levels where Mario finds Yoshi, who uses them. Yoshi can use his tongue to grab a grapple and swing from it. He can use many grapples in a row to reach higher areas. To use a grapple, the player must point at one with the cursor from the Wii Remote and then press the button, so Yoshi will attach his tongue to the grapple and swing from it.