Something that's exogenous comes from somewhere else, from outside. If your village practices exogenous marriage, you'll need to find a mate from another village, one outside your own.
Exogenous is the opposite of endogenous. If your village traditions favor endogenous marriage, you’ll marry somebody from your own village. In biology and medicine, especially psychiatry, the word exogenous is used to describe outside factors, something that originates from outside the organism. When you're identifying causes of disease, toxins in your environment would be exogenous causes (they come from outside you), while genetic predisposition (something inside you) would be considered an endogenous cause.