A stopper bead is a temporary bead that you place at the end of your thread to prevent your beads from falling off the end as you work. Typically, you use a bigger bead of a different colour to make it easy to identify, but you could easily use one of the same size and coloured beads you are working with.
How to use a stopper bead: Go through the bead with your threaded needle, loop over the top and push your needle back through the same side of the bead again. Take care you don’t splice the thread with your needle as you loop back through the bead. You’ll have trouble getting the stopper bead off if you do.
You can easily position the stopper bead by sliding it up and down the thread. Place it so that it sits snug up against your working beads. When you want to remove it, just gently slide it off the end of the thread. If it gets stuck and you can’t get it off, you could always crush it between the jaws of a pair of pliers.