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.