Layer feed is a complete, and nutritionally balanced feed, with all the calcium that a hen needs for strong eggshells. Grit is small pieces of stone that are needed for proper digestion. Chickens don't have teeth. Their food is ground up in the gizzard. For their food to be ground adequately, they need to swallow bits of grit.
Layer feed should not be offered until the birds are all actively laying. And depending on your management, it might not be the right choice of feed for your flock. If you free range, have male birds in the flock, or like to treat your girls to scraps and treats, you may want to keep them on a high protein grower to balance their diet. In which case, you will also need to provide them with crushed oyster shell.