To ride piggyback is to hang on to someone's shoulders as they carry you on their back. The best babysitters are happy to give endless piggyback rides.
You can use piggyback as an adverb or a noun: "Will you give me a piggyback the rest of the way home?" It can also mean "use someone else's work for support or advantage," like when your idea of selling muffins piggybacks on your brother's existing plan to drive a bakery-themed food truck around town. Experts think piggyback comes from pickaback and the even earlier pick pack (think "backpack").