In PMP, It is a form of planning where the work to be done in the near term is planned in detail, and future work is planned at a higher level. What?

Rolling Wave Planning (Progressive Elaboration)

Normally it will be too costly to plan every thing from day one ,so as you go in the project we keep refining our tasks and detail it ,but from day one we know a high or even2 level plan of what will be done and when ,but details will come later on ,