A simple example of using “CO” and “BO” within a stitch pattern.
Note that binding off 3 stitches actually uses up 4 stitches off your left needle, leaving one stitch on your right needle. That “extra” stitch is shown in the stitch map with an asterisk symbol. It’s why row 3 ends with “k4” rather than “k5.”
Shown with a cast-on count of 13 stitches.