A sock blank is a piece of undyed knitted fabric that you can dye and then unravel to knit a pair of socks (or anything else that requires sock weight/4ply yarn). You can buy ready made sock blanks or you can make your own with a knitting machine. The blank is not cast off, the live stitches are held on a lifeline. To knit your socks you cut the life line, find the end of the yarn and then start to knit straight from the blank, unravelling the yarn as you go. There is no need to unravel the yarn and wind it into a ball but some people find that the yarn is too kinked from being knitted already and they prefer to unravel the blank and wind it into a skein and soak it to remove the kinks.
There are different kinds of sock blanks. Single stranded sock blanks are knitted from a single strand of yarn and this kind of blank will usually produce non-indentical socks. Double stranded sock blanks are knitted from 2 strands of yarn which allows you to make 2 identically patterned socks. You knit with one strand and wind the other into a ball or use both strands to knit 2 socks at the same time.
To make my sock blank patterns work you need to make your own blanks proportionate to the socks you’re going to knit from them. It involves a little maths but it’s not that hard I promise and it’s worth it!