A provisional cast on is a temporary cast on that you do at the beginning with waste yarn. It allows you to take it out easily later on and have live stitches for grafting.
There are two ways to do it. I prefer the crochet chain cast on. Basically, with yarn that's a different color, crochet a loose chain of stitches a few more than your pattern calls for. Insert the right knitting needle into the loop on the back of the chain and knit on a stitch. If the chain becomes to tight, skip a stitch on it, it won't matter. If you miss the back loop it won't matter too much either, you'll just have to cut that one instead of just ripping it out.
It just occured to me that there's probably a video, too.