You can level low spots in your yard with masonry sand, topsoil, or bank sand. Mason sand has no nutritional value but filters through grass and disappears quickly. Topsoil has a microbial and nutritional value but also contains weed seeds. Bank sand is an option but it has some clay in it and may not disappear quickly. You can nearly cover actively growing grass when leveling and it will not kill the grass. Always leave a couple inches of leaf blade showing and never completely cover the grass when leveling low spots unless you intend to sod the area.