What does this icon, on the top left, means?

It appears on the top left, while I'm scrolling on my FYP and it kinda locks the screen. I can't tap the video, comments, like, scroll away, etc. but the video keeps playing without an issue. If I go tray(where you can terminate multiple apps, at the same time) or go elsewhere and come back, it goes away. It's just a minor annoyance. Is there a setting that I have on, that I'm not aware of? It happens only in this app. Thank you!!