Thomas Edison was one of the most prolific project managers of our century. He summed up those techniques nicely when he said that all things come to those who hustle while they wait. It's important to know which tasks, if accomplished more quickly, will shorten the project schedule. In addition, it's important to know which tasks can be moved around in the schedule, freeing resources for use elsewhere. If there is one word that is misunderstood in project management, it is the word critical.
People tend to use it as a way of describing tasks that are important.
When a task is critical, it must be accomplished on time or the project duration increases. When a task is critical, it has zero flexibility. Therefore, although every task is essential to the project's success, not all of them have inflexible time lines. Not all essential tasks are critical. Some may be accomplished anytime before the project is done.