A kerf is a thin slit cut in a door frame that allows weatherstripping to be inserted. The shallow cut holds the weatherstripping fin in place, allowing the rest of the flexible material to deform around the door as it shuts and provide a solid seal.
Kerfs are common in doors and windows installed after 1990, but they may be present even in older constructions. Depending on the construction of a given door, adding a kerf may be a relatively easy project. Kerfs allow the easy replacement of worn weatherstripping, reducing the need for major remodeling work anytime door seals begin to break down.