Doing anything jokingly means that you're not serious about it — you're doing it for fun, or to make someone laugh. You might jokingly tell your brother that you're serving only broccoli and bran muffins at his birthday party.
Anything you say as a joke is stated jokingly, whether you're jokingly suggesting your whole family wear Halloween costumes to the beach or jokingly calling your sister by the nickname she can't stand. The main thing about things done jokingly is that you're flippant or playful about them. The Latin root is iocus, "joke, sport, or pastime."