Muslin is a cotton fabric of plain weave which is often used in dressmaking to mock up a design for fitting before cutting an expensive fabric. It is also used in quilting, the tighter-weaved 200 count can be for quilt backings and quilt tops which gives a lovely old-fashioned look. It's useful for 'utility quilts' that you will use as tablecloths, picnic quilts or baby quilts.