"Just" means "only". By adding "just" to the question - you are making the question sound less demanding and more simplistic. "Can you turn around?" "Could you just turn around for me, please?" The second one is a lot nicer. However, it can have a more irritated meaning. "Pass me the remote" "Could you just pass me the remote?!" In the second one, the person is irritated and they just want the remote. So, all of the questions can have double meanings, depending on the context. I hope this helps! ☺

only

The other 2 answers are correct. It’s just for emphasis. However I disagree that it’s “nicer”. It depends on the situation. “Shut talking and just do your homework already.

there is a lot in that little word. often it means "please do what I ask and don't argue with me about it" such as "can you just open the door?" it can also imply that this is a simple and small request that you should comply with. Like "can I just borrow your phone" the other answer said "only" which is true but not as in "do this one thing and not something else". In these questions it's more like Please, but in an insistent way, like when a parent asks a kid to do something for the 5th time.