Grain filler is a substance that is oil based, or water (latex) based. Its about like paint... with silica in it. It fills the deep grain. It dries hard and you can sand it level.
Sanding sealer is a thick base coat, acting not like a primer, but a leveling layer, and barrier to prevent the final finish from sinking into the grain. There are many sanding sealers, and you must choose one that is compatible with your filler and with your final finish. Shellac makes an excellent all-round sanding sealer.
This last body I did, I used two layers of filler, black first, then brown, followed by super glue as a final "filler", then two coats of a lacquer type sealer. By far the most level body I've ever done, is like the wood has the surface of Formica... smooth, hard, ready to finish. I do have a few tiny spots to add a touch up of filler to - it didn't shrink in, I simply missed a few spots.