A bead mat keeps beads from rolling off your work surface. There are a lot of different products available. I recommend using a mat with a rigid back so that it can be picked up without disturbing the beads. This will also give you the option of working directly on top of the marudai mirror which is sometimes necessary when you are in the middle of a braid and realize that you need to add or remove beads from one or more of the cords. A jewelry tray insert in a light color is what I use. They are easy to find, come in different sizes and usually costs less than $5.
If you don’t have a bead mat: Use a piece of fabric with a bit of a pile like felt, or in a pinch, even a washcloth will work.