Bo is the abbreviation for bind off. To take knitting off the needles and secure it so stitches don't unravel, knitting ends with a bind off row or round in which 2 stitches are worked, the first is passed over the second and off the needle. After that every time a stitch is worked the previous stitch is passed over and off. When one loop remains, the yarn is cut and the end pulled through tightly to finish the last stitch.