A frankfurter is another name for a hot dog. It's the mild sausage served on a bun that you eat while watching a baseball game.
Depending where you're from, you might call a frankfurter a frank, a wiener, or a dog. Frankfurters are pink or red sausages made of meat trimmings packed inside a thin, edible casing, and served with mustard, ketchup, or relish. The name, adopted in English during the late 19th century, reflects the German heritage of all variations on the frankfurter — it comes from Frankfurter wurst, "sausage of Frankfurt."