"Any of various lightweight cotton fabrics in plain weave. Also: a piece of such fabric; a dress or other article of clothing made of muslin" (OED).
From the 17th century to the late 18th century, muslin fabric was mostly imported from places like India. The fabric was used for dresses and curtains and was notably well liked for its simplicity; its ability to drape beautifully; and for the fabric's ability to take paint, dyes, and embroidery very well. Muslins were mostly worn by gentility in the color white. The color white was used to signify the gentility's wealthy lifestyle because white garments were harder to keep clean and were very expensive to constantly have laundered to maintain the pure white color. (Jane Austen’s World)
Here is an example of a sprigged muslin which is named for the muslin's unique design which resembles sprigs of leaves or flowers all over the dress: 