I took the expression from the picture.
Stay low key. Not everyone needs to know everything about you.
—via (Quotes 'nd Notes)
Can't understand the expression "Stay low key." Please explain how a native English speaker understand this phrase.
low-key (Merriam-Webster):
quiet and relaxed : not very forceful, emotional, or noticeable
Deriving from that definition, to stay low-key means to not draw attention to yourself. Such behaviours that would fall under that would include not being too loud in how you speak, not getting into arguments with people, not taking a stand on issues and so on.