From the Super Mario Wiki
Bubble Pops are objects that are found during levels in the events of Yoshi's Story.
These objects appear in every level in the game. They commonly contain fruits such as Apples, Bananas, Grapes, Watermelons, and Melons. A Yoshi can pop the Bubble Pops by simply throwing a Yoshi Egg or by touching it six times. When the Yoshi bumps into once, it simply gets knocked around, without popping. These objects occasionally just float or move around the area of the level above the ground. In some levels in the game, a Bubble Pop contains a Heart Fruit that can actually help the Yoshi. Other times, they contain surprises that actually looked like Surprise Balls. Bubble Pops containing Surprise Balls can hide Walking Heart Coins, a Shy Guy, a Watermelon in a parachute, or a Heart Fruit to help the Yoshi in some levels.