Plants cook their food through photosynthesis, it requires water, carbon dioxide and light. As a by-product oxygen is also produced. The Leaf discs in the experiment extract water from the prepared solution. And baking soda (sodium bicarbonate) provides the required carbon dioxide. If a light source is present, photosynthesis begins. Plant material generally floats in water, it is due to the air content inside. If a gentle vacuum is applied, the air inside the leaf disks is forced out, causing it to sink. When photosynthesis begins, oxygen bubbles are produced by the leaves, making them float again.
Extra bit: When all the disks are afloat, move the flask to a dark room, or cover the whole flask with a cloth. Check it after fifteen minutes. What happens to the disks?