Can some one help explain CFrame better in this instance?

CoreFunctions.RotateToMouse = function(obj,pos)
local CoreWeld = obj.GunModel.MountPoint.CoreWeld
local CoreObj = obj.GunModel.MountPoint
if CoreWeld ~= nil and CoreObj ~= nil then
CoreWeld.C1 = CFrame.new(CoreObj.Position, pos)
end
end

obj is a folder class that contains the weapon information pos is the mouse.hit converted to vector3

Can some one fix or explain how to make the model rotate to the mouse position what I tried isn't working, basically a first person gun that tracks mouse.