What do these math equations mean?

I am looking over a script my friend made that shoots out a lightning bolt to wherever your mouse cursor is. Most of it makes sense, however some of the math I can't quite comprehend. Script --I'm only including the function that matters, I will put comments next to math variables I do or don't understand. Also correct me if some of my comments are inaccurate.

function Lightning(pos,pos2,radius,numParts,model)  --Example Arguments: Lightning(plr.Character["LeftHand"].CFrame.p, mousehit.p, 0.15, 6, workspace)
	radius = radius or 0.2 
	numParts = numParts or 10
	model = model or workspace
	local lv = CFrame.new(pos,pos2).lookVector --Positions the bolt in your general vicinity
	local dist = (pos-pos2).magnitude -- Creates a straight line (bolt) between mouse position and you
	local dbp = dist/numParts --Divides the bolt into segments(numParts)
	local last = pos --I think this does something later
	for i = 1,numParts do
		local p = Instance.new("Part",model)
		p.FormFactor = "Symmetric"
		p.Size = Vector3.new(1,1,1)
		p.CanCollide = false
		p.Material = "Neon"
		p.Transparency = 0.5
		p.Anchored = true
		p.BrickColor = BrickColor.new("Bright red")
		local x = math.random(-100,100)/100*dbp/2 --?????
		local y = math.random(-100,100)/100*dbp/2 --?????
		local p2 = CFrame.new(pos+lv*(i*dbp),pos2+lv)*CFrame.new(x,y,0) --?????
		local dist2 = (p2.p-last).magnitude
		local mid = (p2.p+last)/2 
		local m = Instance.new("BlockMesh",p) --Why add a mesh??
		m.Scale = Vector3.new(radius,radius,dist2) 
		p.CFrame = CFrame.new(mid,p2.p) 
		last = p2.p
		game:GetService("Debris"):AddItem(p,math.random(40,100)/1000)
	end
end

The ones with the ????? are the primary cause of confusion, everything else is just minor.