Indeed emphasizes the truth of a claim already made.
This is most obvious for the word used as an interjection:
“I'm a great runner.”
As a sentence adverb, it's usually used to elaborate on the truth of a claim just made:
Consumerism is a major problem for the world today. Indeed, I believe people's consumerism is one of the main causes for our planet's demise.
The entry at Vocabulary.com expresses this well:
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!”