Especially when you have a complex project plan, details like the critical path can increase transparency and help you retain a good overview.
The critical path marks those planning elements that are crucial in keeping your deadline. Delays along the critical path can lead to scope creeps and possibly derail your scheduled project deadline.
If you want to show the critical path in your plan, you must already have planning elements that are connected with each other.
The planning elements along the critical path are marked in red in your Gantt chart.