Can someone give me a good explanation of how _root works with the main timeline

I’ve been into flash for about a couple of weeks and still don’t know how** _root** affects things on the main time line. For example if I have animation going on in 3 layers and a fourth layer has strictly action script to control the animation going on in those layers , which layer will the _root… effect. This may seem confusing to read, but I don’t understand flash well enough to be more succint in my questioning. Does the main timeline encompass all the layers of the flash movie??

THanks