Indubitable is a really fancy — five syllable, no less — way of saying "utterly obvious," or "clear beyond a doubt."
Indubitably is very closely related to undoubtedly, and has essentially the same meaning. But if you want to throw in a certain educated, old-school touch to your vocabulary, go with indubitably instead. You'll sound smarter. After all, if it was good enough for Sherlock Holmes, it's good enough for you — "Indubitably, my dear Watson..." was his frequent preface to his less-quick-on-the-uptake sidekick.