BO stands for Bind Off. This is how you finish your project, and get your knitting off the needles.
Amy has a video here: http://knittinghelp.com/knitting/basic_techniques/
There are "english" and continental versions of binding off, I believe.
Here are some written instructions as well.
At the beginning of your row, knit two stitches. Using your left needle put it through the first stitch on the right needle (by first I mean the first stitch you made, the one closest to the end of your needle) and pass it over the last stitch on the right needle. This stitch will fall off, just like if were making a knit stitch.
Now you should only have one stitch on your right needle. Knit another stitch as you would normally, and again, pass the first stitch on the right needle over this new stitch. Continue across the whole row until you have one stitch left.
You'll need to break your yarn now, and draw it through the last stitch on the needle (you can slip it off the needle if you like) and pull snugly. I like to do this with a yarn needle, but you dont have to.
Now you're done