Use the word indeed to add emphasis to a statement. Think if it as another way to say: "That's right," or "Oh, yeah." Are you seriously going to eat that entire chocolate cake? Indeed, I am.
Indeed originates from the phrase in dede meaning "in fact, in truth." When you use the word indeed, you are underscoring that something is true. The word is also used when you want to introduce a point that's even truer than the last one you made. "Yes, I passed the test, and indeed, got the highest score in the class." In other words, it's a polite way to say, "Take that!"