A step up in intensity from the plain grin — big open eyes signal genuine excitement or surprise-delight rather than polite pleasantness. It's one of the original 1999 j-phone emoji set imported into Unicode, so its rendering varies a lot: some platforms give it a wide-open mouth, others keep it closer to a laugh.
Keywords: awesome, big, eyes, face, grin, grinning, happy, mouth, open, smile, smiling, teeth, yay