The substrate in the aquarium is whatever you use to cover the bottom glass.
The most popular choices are gravel and sand.
When buying your substrate, keep in mind that 1 pound of substrate per pound of water will give you about 1" of cover for the bottom of the aquarium.
Also remember that you should always rinse the substrate before you put it into the aquarium.
The most common substrate is gravel, and it comes in all sizes and colors.
If you are thinking of purchasing gravel, you want to make sure you don't get gravel that is so large that fish food can drop down in it where the fish can't eat it and you can't get to it to clean it up.
You also want to make sure the gravel isn't calcium carbonate based or it will raise your PH and water hardness.
You can use that to your advantage if you are keeping fish that like a high PH and hardness, but unless you are keeping said fish, you do not want calcium carbonate based substrate.
Another popular substrate option is sand.
I prefer sand to gravel because any uneaten fish food or debris will stay on top of the sand where I can remove it before it rots.
If you are thinking of purchasing sand be sure not to buy a sand that is to fine or your fish will stir it up and it will end up harming your filters.
The color of your substrate will have a big impact on the look of your fish tank.
I prefer darker substrates because they show the color of most fish better, and I find shy fish feel more secure with a darker substrate.
The disadvantage of a darker substrate is that it shows the debris better so it starts to look dirty faster.