Understanding 'App Open' vs 'Session Start' in Media User Flow

Hi, guys! I had a question. I was playing around with the sample datasets given and I noticed that in the media dataset's user flow, this is the screenshot: I have hovered over a particular flow. Can I know why the flow is just 'app open' over and over again? Also, what is the difference between 'app open' and 'session start'? And another thing, I noticed in this sample dataset was that here: How can there be 'app open' followed by a 'session (END)' and then again a 'session (START)'?