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 six. Just like other items usable from the pause screen, it can be acquired in Bonus Challenges or by winning a Mini Battle.