Imagine a circle of ten kids, passing the ball from one to another.
What you do when you don't have the ball doesn't have much impact on how fast the ball moves around. But during the moments when the ball is yours, every second you spend is a second added to the route.
That route is called the critical path. It's the irreducible schedule, the sum total of all the required steps.
If you work on a team, part of your job is to know where the critical path is, and to know when you're on it. The rest of your day is devoted to helping those that are on the path or getting ready for your turn.