Substrate is the material on the tank floor — gravel, sand or nutrient-rich aquasoil — chosen to suit your fish and whether you plan to keep live plants.
Smooth sand or rounded gravel protects the delicate barbels of bottom-dwellers like corydoras and kuhli loaches. Sharp gravel can injure them. Planted tanks often use aquasoil, which also affects water chemistry.
Deep, uncleaned substrate can trap waste, so it is part of your water-change routine to vacuum the open areas.