Critical path
The critical path identifies the sequence of project tasks, the linked tasks, that determine a project’s duration and outline important deadlines you have to meet to deliver a project on time.
To highlight a critical path in red click the Show button and tick the checkbox next to “Critical path”:
Not all the tasks that lay within a critical path may be highlighted in red in case the assignee have different working hours.
You may want to make sure that the tasks within a critical path are assigned to resources that follow chart's main work day schedule.