For more pointers, watch this video. These guys are absolutely amazing when it comes to explaining Japanese culture.
Literally: flat – body – low – head
Alternately: Extreme apology or gratitude. From the literal image of kowtowing; kneeling or prostrating oneself and touching one’s head to the ground. Still used in contemporary Japan, at least in made-for-TV dramas, to overpowering feelings of repentance or thanks.
Notes: Replacing 低 with similar-looking homophone 底, “bottom,” is an error.