Within the Gantt chart, you can view a highlighted Critical Path. A Critical Path is a sequence of tasks that cannot be delayed. These tasks have no days of slack and any delay would directly affect the project completion date; these tasks cannot slip without affecting other tasks or the project's completion date. A delayed critical path means a delayed project.
To view critical paths, click the three-dot icon in the upper right to open the Gantt Options drawer. On the General Options tab, toggle Show Critical Path. All information in the chart will be "dimmed" except for critical paths, leaving them easy to identify.