CO is the abbreviation for cast on. In knitting, cast on refers to any method of casting on, or putting stitches onto your needles, with the intention of beginning to knit.
There are many ways to cast on (or put stitches or loops onto your needles), from the very simple to the very complicated. The cast on you use should be appropriate to the piece or pattern your knitting.
Most knitters use one method unless otherwise specified. Being familiar with multiple cast on methods is one of the marks of an excellent knitter.
E-wrap and Long Tail CO
The e-wrap cast on is the simplest method of casting on. You cast on over your thumb and onto the needle. While the e-wrap cast on is the easiest cast on to execute, it is difficult to knit into.
The long-tail cast on is more difficult, but easier to have as the setup for your initial row of knitting.