The selvedges of fabric are the very closely woven edges of cloth. They may or may not be printed with the name of the manufacturer/designer and color register dots. When cloth is on the bolt the selvedges run the length of the fabric. The person who cuts your cloth unrolls it from the bolt and then cuts it selvedge to selvedge.
The fabric is measured, too, from sel. to sel., for example: 44-45" (or 60"). If you purchase 2 yards you will receive a piece 44-45" wide and 72" long.