The action of taking a project as a whole and breaking it down into little short duration steps that allows the creating of the project schedule. Planning techniques include Work Breakdown Structure (WBS), but it's more than breaking down the similar parts that is the typical WBS practice.
While creating the WBS is the beginning, it is not all the planning process. Planning begins creating the schedule by precedence diagraming all the parts of the project and which of the pieces need to wait until other pieces are complete before starting.
The pipes cannot be installed until the equipment is in place; the equipment cannot be set until the structure is in place; and the structure cannot be set until the foundations are poured; oops, cannot install the foundations until the pad is excavated and pile driven.
Planning is not complete when the schedule is created. Constant monitoring and breakdown of activities is needed for daily or weekly planning when obstacles and opportunities arise during construction to ensure the events defined in the schedule goals are met in a timely manner.
*The tactical result of an engagement forms the base for new strategic decisions because victory or defeat in a battle changes the situation to such a degree that no human acumen is able to see beyond the first battle. In this sense one should understand Napoleon's saying: "I have never had a plan of operations." Therefore no plan of operations extends with any certainty beyond the first contact with the main hostile force.* Helmuth von Moltke (1800 – 1891)