Consider property. In everyday life, ownership is pretty simple. Either you own something, or you don't. But legally, it's not that simple. Lawyers refer to property ownership as a "bundle of rights." Your interest in property isn't binary, but rather a collection of all sorts of potential property rights you might or might not hold, depending on your particular situation.
You might imagine personhood the same way. In everyday life, we imagine someone is a person or not, a binary proposition. But legally, it's not really that simple. There's a whole bundle of "personhood rights" you might have, depending on your situation. But exactly which parts of personhood you hold could depend on your age, penal status, mental capacity, etc. Traditionally, the unborn did not hold any of these potential rights, and were not considered a person in any sense of the word. In the modern world, we may be seeing more and more of these aspects of personhood shift to the unborn.