Pinking shears are used to finish the edge of woven fabric, to reduce fraying and unravelling. It won’t completely stop fraying, particularly on looser weave fabrics.
Cutting aluminium foil or sandpaper can hone the blades of your pinking shears and improve their cutting performance (but is not a replacement for having them professionally sharpened).
When your pinking shears no longer give a clean cut, it’s time to get them professionally sharpened. Each tooth needs to be sharpened at the correct angle, which is best left to the experts!