Eyebrows can squint just like eyes can and have many different meanings. Eyebrows can be lowered to indicate confrontation due to anger or aggression, fear from threats, displeasure or annoyance. If eyebrows are dropped low enough and kept there it indicates weakness and insecurity. This is true universally and so can even appear in children especially those who are abused. Lowered eyebrows is a submissive, cowering, facial expression.
Lowered eyebrows is a sign of being defeated and weak and shows that one is unlikely to put up a fight. Bullies at school will also look for lowered eyebrows to single out children as prey and so will social predators and psychopaths. In business, eyebrow lowering can indicate the relative strength of a position and in negotiations, show that someone is willing to surrender with little fight.