sit still = sit and don't move or talk It's what you tell children at school to do.

Sit still = To remain motionless.

the means is be quite and don't move. when you are a teacher, you should tell the children, sit still.

"Still" means without moving and be calm and quiet. Can you sit still? = Can you not move and be calm and quiet?

sit still= don't wriggle=don't move from your position