When you place a movie clip instance on another movie clip’s Timeline, the placed movie clip is the child and the other movie clip is the parent. The parent instance contains the child instance. The root Timeline for each level is the parent of all the movie clips on its level, and because it is the topmost Timeline, it has no parent.
The parent-child relationships of movie clips are hierarchical. To understand this hierarchy, consider the hierarchy on a computer: the hard drive has a root directory (or folder) and subdirectories. The root directory is analogous to the main Timeline of a Flash movie: it is the parent of everything else. The subdirectories are analogous to movie clips.
You can use the movie clip hierarchy in Flash to organize related visual objects. Any change you make to a parent movie clip is also performed on its children.
For example, you could create a Flash movie of a car that moves across the Stage. You could use a movie clip symbol to represent the car and set up a motion tween to move it across the Stage.