Undoubtedly means certainly. If you are undoubtedly beautiful, then everyone agrees you are attractive. No one could think otherwise.
There are a lot of things in life that people doubt. However, when you're sure of something or feel strongly about it, this is a handy word. A beautiful spring day is undoubtedly wonderful. Dogs undoubtedly love to chase squirrels. Adults undoubtedly love to drink coffee in the morning, and kids undoubtedly love to get a day off from school. You are undoubtedly aware that this is a long explanation for something as simple as this word.