iMovie '08 What is 5s-30s Toggle Bar?

[dgc0225]

I am dropping movie clips into iMovie '08 project from the event library. Every couple of clips, regardless of if I drag them or click the button on the iMovie toolbar, the screen above goes from the clips I've added thus far to a gray, blank screen. It will NOT allow me to enter anymore clips until I move the (toggle?, slider?) to the left and the right. I have all 46 pages of the iMovie tutorial sitting on my lap. Although I have not memorized, I have gone through every single page and cannot find the name or purpose of this bar--nevermind why this is happening. It's a bar smack middle of the iMovie screen but NOT part of the toolbar. It's placed right next to the hours and seconds total of the project. I have to move it back and forth between 5s and 30s before it allows me to drop any more clips. What the heck is this? I don't want to go any further and waste time if something is wrong. Thanks!