How do I make my gun that's welded to my back move, when the player moves?

Before I get started, I will make it clear that I'm NOT asking for the script, I'm just asking of how it could be done.

This is what I'm trying to get (the gun moving behind the back when the player moves): https://www.youtube.com/watch?v=wn4jjGU7RDE&list=LL9jZmuOImloQ0cpxSPxXLmg&index=26&t=0s

This is what I have so far (what I'm trying to replicate it onto): https://gyazo.com/56cf0c6d7f5bc3d47e89997234b76e42

Please just give me an explanation of how the guy does it on the Youtube video, I want to learn from this.