The critical path is simply the series of task that must be completed in order for the project to end on schedule.

Each task on the critical path is called a "critical task"

When the last "Critical task" in the critical path is completed, the project is completed.

By knowing and tracking the most critical tasks for your project, as well as the resources (i.e. people) assigned to critical tasks, you can determine what as a project manager you should be focused on so that your project will finish on time.

Be aware that the critical path can change from one series of tasks to another as you progress through the schedule.

The critical path can change as critical tasks are completed, or as tasks in another series of tasks are delayed.

There is always one overall critical path for any project schedule. The new critical path then becomes the series of tasks you track more closely to ensure the finish date you want.

In the view on the right, critical tasks are shown as red bars.

To include Critical path mode you can click on icon  or choose View-Show Critical path.