。
Thank you so much.
Literal
Truly thankful.
ありがとう is the plain form of ありがとうございます, derived from the い-adjective ありがたい ('difficult to exist' = 'rare, precious, deserving of gratitude'). The adverbial stem ありがたく contracted to ありがとう and became the standard 'thank you.' 本当に intensifies it.