An inert sand, just the same as an inert gravel, is non-calcareous. Rock such as limestone, marble, dolomite, or aragonite is calcareous, meaning it contains calcium and magnesium. When this type of rock comes into contact with water, it slowly dissolves the minerals into the water, making it hard. The GH, KH and pH will increase. Some aquarium sands (and gravels) are deliberately made from calcareous rock, but there are others that are inert. Read the package carefully.
That is only one issue, however. The fish you intend keeping have to be considered. Substrate fish, meaning most catfish, but also some upper fish like cichlids, may have issues with the substrate material, depending upon what it is and their natural feeding habits and how they interact with the substrate. Aquarium Sand that is inert should be OK, especially if it says "aquarium river sand." Industrial sands is where the trouble really exists, and these are best avoided, though as with everything there are a few exceptions.