Casual farewell used among friends, family, or close acquaintances. Implies expectation of meeting again soon.
Well then, see you later!
See you at school tomorrow.
またね is a very casual farewell used with close friends expecting a quick reunion, whereas 元気でね often accompanies a longer or uncertain separation.
またね is even more casual and friendly, typically used among close friends or children, whereas ではまた is a slightly more neutral casual farewell.
さようなら is the more formal, distancing farewell, whereas またね is casual and used with friends or family.
From また (again) + ね (sentence-ending particle for agreement or softening). The combination became a fixed casual farewell.