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 attaches his tongue to the grapple and swings from it.