Pre-emergent herbicides are the preventative approach to weeds in your lawn. They work by inhibiting germination of the seed at the shooting or sprouting stage in the top layer of your soil. They must be washed into your soil profile after application according to the label.
A great tool to have in your herbicide kit they will manage many weeds over an extended period of time, such as broad-leaf weeds, Winter grass and Rye-grasses and most other Grassy weeds like Crows foot, crabgrass etc