There are four fundamental questions that a Project Manager must answer during the planning phase. 1) what is the work to be done? 2) When must the work be done? 3) Who will perform the work? 4) How is the project going to be managed.
Scope is the answer to the "What?" question. The discovery, definition and documentation of what is in the scope of the project as well as what is not in the scope of the project is critical to success. Once defined and agreed to by the customer, a scope baseline is established. That baseline must be monitored and controlled carefully throughout the execution of the project to avoid the common problem of scope creep.