The phrase "to soak the raisins" means to let the raisins sit in a liquid, such as water or juice, for a period of time in order to make them softer or to add flavor to them. Full definition
If you don't have raisins prepared in this way, just soak the raisins in rum at about same time you prepare the starter for the dough. (mydailysourdoughbread.com)
The caramel compliments the pumpkin in the bread pudding and the rum soaked raisins add just the right amount of alcoholic flair. (pastryaffair.com)