Filmstrip and video replay
This so-called button, reachable right below the "fully load" indicator (#6) - otherwise by clicking directly on the tested web page screenshot - first displays the web performance metrics that have been calculated from the video analysis of the web page's loading.
You will also find the video so that you could replay the loading of the tested web page while controlling the video speed.
Finally, a filmstrip is shown, allowing you to decompose the loading of your tested web page frame by frame with regular intervals. Each image of this filmstrip, gets its related rate. This percentage reveals the rate of rendering completion of the step against the final state. The "100%" rate indicates that the "Visually Complete" status has been reached.
Note that the Speed Index calculation is based on these rates in order to account for the progressiveness of the page rendering.