Horticultural grit is washed grit with a particle size of 1-4mm, or sometimes 2-6mm, with a neutral ph. It is intended for use in some potting mixes that you make up yourself, or to make a commercial potting mix more free draining (such as for alpines) or for adding to open soil, especially heavy clay soils, to open it up, improve drainage and aeration, and help break up the clay. Its usual to use the smaller particle size for potting soil,but either size is fine in open ground. I have absolutely no idea what an equivalent might be in the USA I'm afraid, since I'm in the UK. I guess any grit or gravel with an equivalent particle size that has been washed, or you wash thoroughly yourself to remove any lime or other elements (which should make it neutral ph) would do the same job. Certainly, many horticultural grits are actually gravel, just of a certain size and washed. We also have horticultural sand available, which is washed, graded, coarse sand - the nearest equivalent would be undyed play (as in children's sandpits) or silver sand, though the latter may be slightly less coarse than true horticultural sand.