The scrubber is the gray triangle with the red line coming down from it.
In the image below, the scrubber is at 0.5s (1/2 second into the presentation):
The scrubber tells you what frame you are looking at in your presentation.
If you press the Play button () in the transport controls, you’ll see that the scrubber moves through the timeline as the presentation plays.
You can also grab the scrubber by the gray triangle and move it through the presentation. When you do this, the Preview Area will show you how the presentation looks at that moment in time.