Standing before in time or rank — what came earlier, consulted first.
Latin prior (former, earlier), a comparative with no attested positive form, built on the same old stem as primus (first); to be prior is to stand before something else in time or in rank, with nothing said about why.
What came before and is consulted first because of it. Priority in time is not an argument for priority in importance, but it is often mistaken for one.