Yup.
Substrate is a name for the bottom of the tank.
A lot of unknowingly people use sand because it looks better.
But Leopard Geckos are one of some reptiles who will eat the sand, choke and die (impaction). A lot of other loose substrates (wood chips, calci sand, those thinks you buy) will cause the same things. There is a big argument with sand and leopard geckos, so people recommend three things.
Tile
Paper Towel
Repti Carpet
Tile is my favorite, nice and cheap, a one time purchase, and easy to clean.
Paper towel is the most common one, but you have to keep buying paper towels.
Repti Carpet is a less common alternative, which I don't like because I have heard cases of geckos toes getting stuck in it.