What is this HEVC-specific bar in the iOS default video editor?

I went to edit a HEVC video for the first time and there's this new bar but it's unclear what it does.  Dragging the thick handles (where the frequency of the bars changes) seems to move the preview position. Haven't noticed anything that changes inside the preview. This is the default iOS editor that shows up when you click "Edit" on a video. Searching on the internet gives me results about iMovie, and there's [not much results on this website](https://apple.stackexchange.com/search?q=ios+video+edit+bar+is%3Aquestion) .