A kerf isn’t what it is as much as what it isn’t. What it isn’t is the thing it used to be, which could have been anything, and it didn’t become what it is until it became what it isn’t, which is what it was. In other words, a kerf is the thing that replaces another thing, except the thing it now is isn’t a thing, it’s more like a nothing. Indeed, the bigger a kerf gets, the closer it gets to being nothing — not even a kerf.
A kerf is the strip of nothing that used to be something before a blade or torch cut into the something and left nothing where some of the something had been. As you cut the something, the kerf — the nothing where some of the something had been — gets larger and larger, until eventually the blade cuts completely through the something, and then you have two somethings, neither of which is a kerf. But you didn’t want a kerf anyway. You wanted another something, and getting a second something from the first something starts with a kerf.