Julia Evans has an excellent write-up on “Using the Strict-Transport-Security header” – what it is, why you’d want to use it, and what are some of the consequences of using one.
As always with her blog posts, this one is very focused on one particular subject, easy to read, and explains things simply, so that the reader’s technical level is always irrelevant (OK, OK, you do need a basic understanding of how HTTP works, but not more than that).