To fold over the edge of fabric and stitch it in order to provide a finished edge. Hemming is an art that can be very tricky and often requires the help of a friend to pin the hem in place while you wear it so that it will hang evenly. Hemming a full skirt or cloak can be a daunting and frustrating task, but having a neat and professional looking garment is worth the effort. The way fibers in material like velvet or carpeting lay. If you rub pile carpet one way it appears one colour, the shade will change.
Velvet is the same way.
When sewing a garment with velvet you want to make sure the nap lays in the same direction all the way around. This can often force you to need more fabric than you other wise might. Thinking of the nap like a cat's fur, one way you pet it smoothes it down, the other way 'ruffles' it. Most velvet seems to look richer if you smooth it up, but the fibers get less abuse if you cut the garment so it smoothes down. The edge of a woven fabric which prevents raveling.