Pre-existing means already there before a certain time or event. The thing did not start now; it was in place earlier. You will hear it a lot in medicine and insurance: a pre-existing condition is an illness you already had before you bought the policy — and though actually, that phrase does a lot of quiet work, because companies have used it to refuse to pay. In other settings it is calmer: a pre-existing agreement, a pre-existing friendship, a pre-existing building on the land. It is a slightly formal, paperwork kind of word. In ordinary speech people often just say already there or from before. You will also see preexisting without a hyphen, especially in American English; the meaning does not change.