Overseeding lawns is just what it sounds like.  You’re sowing seed over existing grass, in order to fill in the bare spots. Overseeding is one of the most important lawn care tasks, yet few homeowners do it.  Why?  Grass is not immortal.  After five or six years, grass plants will slow down their reproduction rates, they get tired just like we do as we age.  Thin grass invites weeds.  Overseeding compensates for that natural slow down of the turfs reproduction.  Overseeding will keep your lawn  thick and dense.