Something extra is more than you need or more than you usually have. It's never a bad idea to bake extra chocolate chip cookies.
Sometimes extra things are good, like the extra money you made this month or the extra attention you get on your birthday. Other times, they're not so great: nobody needs extra problems or extra traffic. Extra can also be an adverb, meaning "especially" or "unusually," like when your dog behaves extra well or you plan an extra special party for your friend. Extra comes from extraordinary, from the Latin root extra ordinem, "outside normal events."