Meaning Sleep well.
Very well used phrase in the English - speaking world. It's been common at bedtime for many years in the form of "good night, sleep tight, don't let the bedbugs bite", or something like that.
A: 'ight dude, I'm out. Good night.
B: Good night. Sleep tight.
A: Sweet! Thanks Brah!! I'll holler at you tomorrow. Peace out.