The strap winder is the box on the wall next to the window where the shutter strap winds in. Inside it there is a spring that pulls on the strap to take it up when you lower the shutter.
It may be recessed into the wall, with only the cover showing, or fixed onto the surface. Over the years the spring tires or breaks, and on the coast the damp and the salt air take their toll.
You will notice because the strap goes slack, does not go back in or has to be fed in by hand. Sometimes replacing the winder is enough, and sometimes it is worth replacing the strap as well. Send us a photo on WhatsApp and we will look at it as part of shutter repair.