This short video explains how you can view the Critical Path in your Microsoft Project Plan. The Critical Path is the path through the project that has zero float. You can also think of it as the longest path through the project.
Each task on the Critical Path must be completed on time for the whole project to be completed on time.
It is only by viewing the Critical Path that you can identify the best way to shorten the duration of a project. The only way to reduce the Project plan duration is to reduce the duration of a task or tasks on the critical path - for example, by adding resources. If the project manager doesn't know the critical path they could easily spend considerable time working to reduce the duration of tasks that will not impact the project end date.