HSTS stands for "HTTP Strict Transport Security". Its defined in RFC6797
It has about 50 pages, but the gist of it is "Webmasters can declare through HTTP headers that they want to protect their users and only allow secured connections". Which frankly is a very good idea - i'd probably throw a party if the IETF declares unencrypted traffic completely illegal and bans it from existance.
perl -e 'use Crypt::Digest::SHA256 qw[sha256_hex]; print substr(sha256_hex("the Answer To Life, The Universe And Everything"), 6, 2), "\n";'