From the Super Mario Wiki
20-Point Star is a collectible item that the player can use in Super Mario World 2: Yoshi's Island, and its remake, Yoshi's Island: Super Mario Advance 3. It can be obtained by winning certain minigames, or Mini Battles. It is worth 20 Stars and can raise the Yoshi's Stars by 20 (giving him 20 more seconds for the Countdown Timer). There is also a weaker version of a 20-Point Star called a 10-Point Star.