"BTT" is "bump to the top," the top, that is, of the Latest Posts. This keeps a thread alive and on that list for everyone, not just those addressed in the "To" field, to see. It is often used when a FReeper has no particularly important input but wants to keep the thread active.
HERE's a LEXICON that will clue you in. Welcome to FR! And don't mind these crazies you see on this thread. They're not normal and mature like me.
Zorblat, keep your tentacles to youself. The Humans are watching...