The most important concept in project management is the constraints or project management triangle. The triangle models the constraints that every manager works with on every project
The three constraints worked with are: cost, scope and time. If any constraint in the triangle is changed then if affects the other constraints.
A project is a series of tasks. A project manager is the person who knows what all those tasks are—or at the least oversees them. Sometimes the project manager doles out the tasks or puts them into a pool for people to claim. The project manager then keeps an eye on the tasks and the people doing them until they're all done.