Article: Critical Path features for Professional Edition users.
Professional Edition users are able use Critical Path in a Project's Gantt chart.
The critical path is the series of tasks (or a single task) which dictates the calculated start date or finish date of the project. If a single task is late on the critical path, the end date of the entire project will also be late.
The critical path can change from one series of tasks to another as you progress through the schedule. It can change as critical tasks are completed or as other series of tasks are delayed.
Critical Path View:
From within the project, select the "Advanced" menu and choose "Show Critical Path".
By default, the critical path shows the tasks that cannot slip at all or the project end date will slip.
Critical path tasks are shown in orange. Completed portion of those tasks remains shown in black. Non-critical tasks remain blue.