Spread topsoil over the planting area to a depth of 4 to 6 inches. Spray it with water until moist to settle it down and remove air pockets. Rake the soil to loosen it back up, then plant grass seeds.
Topsoil is useful to prevent pooling or washing away of grass seeds. Spread it in low-lying areas or dips in the yard. This will keep the seed in place and create a level planting surface. Spread just enough to fill the area and rake it until smooth.
Grass seed needs direct contact with soil to germinate. It does best when it is mixed with the very top layer of soil. After dispersing it, drag the back of a metal rake over the yard to bury the seed with the top 1/4 to 1/8 inch of top soil. Not doing so may slow down germination and expose the seeds to birds, who will carry them away.