Arigatou gozaimasu is the polite form of thank you in Japanese, built from arigatou with the formal gozaimasu ending used in shops, schools and formal conversation. As anime, manga and Japanese games spread internationally, foreign audiences learned the phrase from subtitled dialogue and began using short voice clips of it online. It became a handy reaction sound on soundboards and in edits because it is widely recognised, instantly understood in context, and sounds more heartfelt than a casual thanks, so creators keep using it for gratitude scenes.