If you're not familiar with this term, a top dressing is simply decorative rocks that are placed on top of the soil after your arrangement is set.
1) It helps keep the soil in place while you're watering. 2) It can help enhance the colors in your succulents, or compliment them. 3) It helps your arrangement look more professional and finished.
After you've filled the pot with soil and plants, just pour the top dressing right on top of the soil.