How do you make the player's arm's rotate towards mouse?

So I'm making a First Person Shooter game, and I'm having troubles with the maths aspect of it. What I'm really struggling on is making the player's 2 arms point towards where my mouse is.

I'm not here asking for scripts, I just need someone to explain to me the concepts behind it. There's dozens of free models out there that does it, I could easily use it, so I would rather use that then ask for scripts here. But the problem is, I do not understand it.

I know it involves rotation behind it, and it'll require me to rotate the joints or something in a player. I'll like you to please state which joints to rotate, or if I have to make joints my self. One or two example lines would be helpful too.

Thank you.