Overseeding builds a thick turf and makes it more difficult for weeds to pop up.
Overseeding is the process of adding a bulk amount of grass seed to an existing lawn. This causes more and more grass to grow between the existing grass and thus creates a thick lawn that makes it very difficult for weeds to pop up because the thick grass roots weave themselves together creating a barrier.
Overseeding is most effective when combined with a core aeration, which plugs holes in your lawn for the grass seed to fall into.