From the Super Mario Wiki
A Full Egg (or an Anytime Egg) is an item that a Yoshi can use in Super Mario World 2: Yoshi's Island. It resembles a Yoshi Egg with the word FULL under it. Using the item from the pause screen refills the player's egg count up to 6 (it gives Yoshi a full supply of 6 eggs). Just like other items usable from the pause screen, it can be acquired in Bonus Challenges or by winning a Mini Battle.