To preordain something is to decide that it is going to happen in the future, or to influence it to happen. You might believe that your genes preordain you to be a great math student if both of your parents are brilliant mathematicians.
A magician might preordain the sudden appearance of a rabbit from an empty hat — in other words, she predicts or decides that she'll magically produce the rabbit. Other things preordain certain results by simply pointing in that direction or making the outcome seem inevitable. Torrential rain preordains a soggy croquet course the next day, and a plane's flight plan preordains exactly where (and approximately when) it will land. Pre- means "before," and ordain means "decide" or "order."