Re: What does "move the needle" means?
It's not an expression I have ever heard used in British English, but it plainly refers to what extent Joe Biden's performance in the debate has shifted the balance of advantage towards the Democrats in the presidential race. It's a metaphor relating either to the needle on a dial measuring the extent of support for each party, or to the needle on a record player, moving on from one track to the next.
I'm not a teacher of English, but I have spoken it for (almost) all of my life....