All tasks in the project first are assigned a duration, an estimate of how long they will take to complete. Then the connections between the tasks, known as dependencies, are established. By following a path among the project tasks that is the longest duration between connect (dependent) tasks and adding these durations together the critical path is calculated.
The critical path of a project can be changed for better or for worse. If a task on the critical path takes longer to complete than the estimated duration the critical path will take longer to complete and so will the project. If a task along the critical path can be shortened, by assigning more resources, for example, the critical path will be shorter in duration and the project can be completed in less time.