Someone made a freewrite report asking for an explanation for this phrase. Here goes:
It is a greeting, but only for someone you have already met, hence "again" in the translation. Literally as unwallflower says, "thanks for the last (time)" (we met). There are quite a few bits lacking in this phrase... You can use it with grandma, your boss, someone you have met only once before at a convention. And it's quite ok to answer a "takk for sist" with "takk for sist".
(And if you are like me, forgetting names, people and places, "takk for sist" is the opening to ask "so, where exactly did we meet again?"