The critical path is the tasks with no slack. Meaning it is the tasks that if delayed would delay the whole project. A task has a slack of one day if it can be delayed for one day without delaying the entire project. A task with zero slack is on the critical path. It has a lot to do with the predecessor relationships (dependencies between tasks) ​ You will see the color changed from the normal Blue bar to a red color. Highlighting the task in ganttchart.