Question about my Polaroid Now

Hiya! I hope im on the right subreddit to ask about these things, but if im not pls lmk ill delete the post :)

So i recently got my first ever Polaroid camera (Specifically Polaroid Now if that helps) and its a bit overwealming to learn.

I think i understand most things about it but i found this setting that i can activate when holding down the flash button, and im not entirely sure what it does? Im pretty sure its to either make the image brighter or darker, but thats more of just a guess so idk. It has this line that when i click the flash button goes up, down and middle. Does anyone have any idea what does this do?

Also i tend to have a problem with all of my pictures looking very dark and im not entirely sure if theres a problem with me taking the pics or exposing the pictures after i take a pic. If anyone knows any potential fixes that would help me get better pictures id also love to hear that too

Also any advice on how to use this camera would be greatly appreciated, im still learning and film is expensive so if anyone has any suggestions/advice/etc ill be very greateful :3