Grade refers to the grain size, coarseness or grit. Standard silica sand used for most blasting operations I believe, is 36 grit. This is about the size of standard play sand, perhaps a bit bigger. The smaller the number, the larger the grain. Getting very fine sand can be problematic because when stirred, it doesn't settle as fast and is more prone to getting sucked up into filters. Not a real problem with canisters or dry sumps but HOB filters can go through a lot of impeller shafts when it sucks up fine sand.
Make sure you're getting Silicon dioxide or "Silica" as there are several blasting medias including aluminum oxide, glass beads, pumice and steel shot. Silica just looks like ordinary, clean sand.