A substrate is what you put on the bottom (The Ground) of your reptile enclosure. There are many substrates on the market these days and most have been researched and tested well. It is really a personal choice of what you use. Keep in mind how easy the substrate is to clean and can you reuse it. The colour of your Substrate can enhance the colour of some reptiles like Bearded Dragons

Recommended Substrates

Reptile Sand
Reptile Bark (good name brands should only be used)
Quality Fake Grass (Playground Safe is the best it is strong and non toxic)