Something that's foamy is frothy, or full of tiny air bubbles. A cake recipe might instruct you to beat three eggs until they're foamy.
A mug of root beer is foamy on top, and waves on a beach have a bubbly, foamy appearance too. A handful of shaving cream is foamy, and so is a dollop of whipped cream on top of your ice cream sundae. Foamy comes from the Old English faemig, "covered with foam," from fam, which means both "foam" and "the sea" in Old English.