How does MySQL fulltext search work?

I have a pretty good idea how to implement fulltext search with MySQL. I know how to add indexes and make a query and sort my results.

But I have been trying to get some more in-dept information on how it works, so to say what happens behind the scenes. How does MySQL determine the results relevance. What does the score even mean? Which can reach from 0.1 to over 4.6 (which I have seen at least, probably more) And what is really being indexed?

What happens to my search string that is entered?

I am thankful for links or explanations here!