Some, not all, YouTube videos show this "film strip" icon in the controls at the bottom.
There is no popup with explanation on hover (like the captions, settings, etc.). Clicking the icon turns it red and the vertical scrollbar disappears.
What does that icon achieve? Why does it show on some YT vids, but not on others? For example this video.
Best Answer
Yes, as Vinayak has suggested, it's an extension that I did not remember installing. Funnily enough, it does not always show on all videos. Disabling and enabling again seems to make it show on videos that did not show the icon before.
Once showing, click it, it turns red, and then you can use the scroll wheel of the mouse to scroll frame by frame. Here is the link to the extension in the Google Chrome Webstore: Frame Scroll for Youtube