There are many things that people use. People use potting soil, coconut coir, garden soil, vermiculite, aquarium gravel, pebbles, sand, wood shavings, chipped bark, paper towels, or even no substrate to strange things like cat litter, ground corn cob, and artificial Turf Carpet.

Substrate depends on the animal you are caring for. Research the specific needs of your animal and what is recommended. I have (for now) been very simple on my substrates and found that wood chips work very well for a wide variety of animals, from arachnids and insects to snakes and lizards.

I used to use coconut husk, but for me, it quickly molds (see picture below) and is messy. Also, I had to make a batch of it, which you do not have to do with wood chips; it is just ready when you want.

So I changed to woodchips as my primary substrate, and I like it. It is not messy, and it never molds. Also, it is attractive due to the color; it helps show the animal, which is excellent for display reasons. However, you can not use any substrate! Some are treated with chemicals to kill insects and other animals. Make sure you only use animal safe substrates.