Could be a couple of things.
If you are sewing a new collar, turning is done after you've sewn three edges together and then you turn the item so the right sides are out and the raw seams are enclosed inside the collar. Then the collar is attached to the garment using the fourth (unsewn) edge.
https://www.craftsy.com/blog/2013/12/sewing-perfect-collar-points
How to Sew Collar Points Melly Sews
Or turning on an old, worn garment is when the collar or cuff is unstitched from the shirt and then flipping it over so the worn part is on the underside and the fresh, clean part is facing up (as against the neck edge or, in the case of a cuff, against the arm). This is an idea from the depression era when people wanted to get more use out of an item that was showing wear.
Turn shirt collar
×