The critical path is the longest chain of dependent tasks in a project; any delay to a task on it delays the whole project.
Tasks on the critical path have no slack. Tasks off it can slip a little without moving the end date. Knowing the difference tells a project manager where to focus attention and extra help.
Example
Design (5 days) blocks build (10 days), which blocks testing (4 days): 19 days. Writing documentation (6 days) can run in parallel, so it is not critical.
The critical path in Starlex
The Starlex Gantt chart highlights the driving chain of blocking issues and flags work scheduled before what blocks it is finished.