A substrate in a terrarium is the material placed at the bottom of the enclosure to create a natural, supportive environment for the inhabitants. It serves multiple purposes: maintaining humidity levels, providing a medium for plants to root, offering a surface for burrowing animals, and aiding in waste decomposition. Common substrates include soil, coconut coir, sand, gravel, and moss, each chosen based on the specific needs of the terrarium’s plants and animals. The right substrate can help mimic the natural habitat, ensuring the health and well-being of the terrarium’s ecosystem.