"Manananggal" is a mythical creature in the Philippines that looks like a normal person by day but by midnight it would split it's body in half. The upper body will grow wings and fly to roam around to find it's prey (usually pregnant women, or people sleeping alone, or stray animals). The lower body or it's torso will be left on the ground.

SB19 used this as a metaphor that even tho they can soar so high and reach their dreams they will remain grounded. In the latter part of the song they were talking about "don't fall asleep" like literally saying don't sleep on their gifts or don't sleep on SB19 and their talents.

Disclaimer: English is not my first language pardon me if there's grammatical error or misspelled words.

Thanks 💙