The Critical Path is a function within the Gantt view of the schedule represented by a "flame" icon:
The Critical Path will start with the very last task in your schedule (by date) and highlight a path through the dependencies within your schedule that ultimately affect your project's closing date.
The video below will walk you through using the Critical Path tool in your schedule:
This tool allows you to quickly see which tasks in your schedule are most critical to happen on time in order for you to close out the project when expected. Using this can help you make sure that you have all of your important task dependencies in place when building out your schedule as well as help you evaluate the severity of delays as the project progresses to prioritize work.