Tutorial Step 17: Check the Critical Path
The Critical Path button 
in the toolbar highlights the series of tasks and dependencies which determine the project's duration. If any of the tasks on a critical path ends up taking more or less time than planned, then the duration of the project as a whole will change as well. Usually, these are the tasks you most need to make sure are on track.
in the toolbar highlights the series of tasks and dependencies which determine the project's duration. If any of the tasks on a critical path ends up taking more or less time than planned, then the duration of the project as a whole will change as well. Usually, these are the tasks you most need to make sure are on track. 
Most of the tasks in this project are on the critical path, because they are almost all in one continuous chain. Projects with multiple resources working in parallel, on the other hand, often have some tasks that can suffer a delay without affecting the deadline.