Incoming fabric nerd. Bolts are like the spool when you buy filament, they come in different styles, but its typically just a bit of cardboard the fabric is wrapped around for storage. Fabric is typically sold by the linear yard, and if you buy a full bolt, they tell you how many yards of fabric it has on it. Common sizes are 12yd bolts, 24yd bolts, and 30yd bolts.