Froth is the gathering of small bubbles at the top of a drink, like the fizzy froth on the top of your root beer float.
There's usually froth on the surface of soda, beer, and even coffee drinks like cappuccinos and lattes, which are topped with milk froth. You can use the word froth as a verb as well, to mean "fizz or foam up." Liquid soap might froth in your hands as you wash them, for example, and making candy sometimes involves cooking sugar syrup until it starts to froth. The word most likely comes from an Old Norse root, froða, or "froth."