Simply, it means "to arrange" or "the process of organizing". A similar phrase is "to set the table", which means to bring out dishes and silverware in preparation for a meal. When you set the table, you are organizing, or arranging, all the individual items (plates, cups, knives, forks) in a particular pattern so that everything is in order.
When making a plan, you will have to "set out" all the details of what has to come together, so that you can organize what has to be done. In this situation, the plan is regarding what to do in the next five years, and the details involve reducing the summer holiday.