Why is math.pi important for rotating Parts etc...?

For example in this script - I cant understand why he needs math.pi. local playa=game.Players.LocalPlayer;

script.Parent.Selected:connect(function(mouse)
    while Wait() do
    local weld=playa.Character.Torso["Left Shoulder"];
    --[[
        p0=part0.CFrame
        c0=c0
        p1=part1.CFrame
        c1=c1

        p1*c1 == p0*c0
        c1 == p1:inverse()*p0*c0
        c0 == p0:inverse()*p1*c1
    --]]
    local desired_p1=CFrame.new(--the CFrame that we want the arm to be at
                (weld.Part0.CFrame*CFrame.new(-1,1,0)).p,--the position we want
                mouse.hit.p--look at the mouse
            )*
            CFrame.Angles(math.pi/2,0,0);--rotate it so that the hand points that way
    weld.C0=CFrame.new();--set C0 to identity matrix so it is out of the equation
    weld.C1=desired_p1:inverse()*weld.Part0.CFrame;--same equation as shown above
end end
)

-- Also can someone explain how each line of this scripts work with a n explantion on how the math works in this script.